Post-translational environmental switch of RadA activity by extein-intein interactions in protein splicing

Improved protein splicing through viral passaging

Intervening proteins (inteins) are translated as subdomains within host proteins and removed through an intein-driven splicing reaction where the flanking sequences (exteins) are joined with a peptide bond. Previously, we developed a self-removing translation reporter for labeling Ebola virus (EBOV). In this reporter, an intein (RadA) containing the fluorescent protein ZsGreen (ZsG) is inserted within the EBOV protein VP30. Upon VP30-RadA-ZsG expression from the viral genome, RadA-ZsG is removed from VP30 through the protein splicing activity of RadA, generating functional, non-tagged VP30 and functional ZsGreen. While incorporation of our VP30-RadA-ZsG fusion reporter into recombinant EBOV (rEBOV-RadA-ZsG) resulted in an infectious virus that expresses ZsG upon infection of cells, this virus displayed a replication defect compared to wild-type EBOV, which might be the result of insufficient RadA splicing. Here, we demonstrate that the serial passaging of rEBOV-RadA-ZsG in human cells led to an increase in replication efficiency compared to unpassaged rEBOV-RadA-ZsG. Sequencing of passaged viruses revealed intein-specific mutations. These mutations improve intein activity in both prokaryotic and eukaryotic systems, as well as in multiple extein contexts. Taken together, our findings offer a novel means to select for inteins with enhanced catalytic properties that appear independent of extein context and expression system.IMPORTANCEIntervening proteins (inteins) are self-removing protein elements that have been utilized to develop a variety of innovative protein engineering technologies. Here, we report the isolation of inteins with improved catalytic activity through viral passaging. Specifically, we inserted a highly active intein within an essential protein of Ebola virus and serially passaged this recombinant virus, which led to intein-specific hyper-activity mutations. The identified mutations showed improved intein activity within both bacterial and eukaryotic expression systems and in multiple extein contexts. These results present a new strategy for developing inteins with improved splicing activity.

Conditional protein splicing of the Mycobacterium tuberculosis RecA intein in its native host

The recA gene, encoding Recombinase A (RecA) is one of three Mycobacterium tuberculosis (Mtb) genes encoding an in-frame intervening protein sequence (intein) that must splice out of precursor host protein to produce functional protein. Ongoing debate about whether inteins function solely as selfish genetic elements or benefit their host cells requires understanding of interplay between inteins and their hosts. We measured environmental effects on native RecA intein splicing within Mtb using a combination of western blots and promoter reporter assays. RecA splicing was stimulated in bacteria exposed to DNA damaging agents or by treatment with copper in hypoxic, but not normoxic, conditions. Spliced RecA was processed by the Mtb proteasome, while free intein was degraded efficiently by other unknown mechanisms. Unspliced precursor protein was not observed within Mtb despite its accumulation during ectopic expression of Mtb recA within E. coli. Surprisingly, Mtb produced free N-extein in some conditions, and ectopic expression of Mtb N-extein activated LexA in E. coli. These results demonstrate that the bacterial environment greatly impacts RecA splicing in Mtb, underscoring the importance of studying intein splicing in native host environments and raising the exciting possibility of intein splicing as a novel regulatory mechanism in Mtb.

An intein-based biosensor to measure protein stability in vivo

Biosensors to measure protein stability in vivo are valuable tools for a variety of applications. Previous work has demonstrated that a tripartite design, whereby a protein of interest (POI) is inserted within a reporter, can link POI stability to reporter activity. Inteins are translated within other proteins and excised in a self-mediated protein splicing reaction. Here, we developed a novel folding biosensor where a POI is inserted within an intein, which is subsequently translated within an antibiotic resistance marker. We showed that protein splicing is required for antibiotic resistance and that housing a stable POI within the intein, compared to an unstable variant, results in a 100,000-fold difference in survival. Further, using a fluorescent protein that matures slowly as the POI, we developed a reporter with two simultaneous readouts for protein folding. Finally, we showed that co-expression of GroEL can significantly increase the activity of both reporters, further verifying that protein folding factors can act on the POI in the biosensor. As a whole, our work provides a new twist on the traditional tripartite approach to measuring protein stability in vivo.

Inteins-mechanism of protein splicing, emerging regulatory roles, and applications in protein engineering

Protein splicing is a posttranslational process in which an intein segment excises itself from two flanking peptides, referred to as exteins. In the native context, protein splicing results in two separate protein products coupled to the activation of the intein-containing host protein. Inteins are generally described as either full-length inteins, mini-inteins or split inteins, which are differentiated by their genetic structure and features. Inteins can also be divided into three classes based on their splicing mechanisms, which differ in the location of conserved residues that mediate the splicing pathway. Although inteins were once thought to be selfish genetic elements, recent evidence suggests that inteins may confer a genetic advantage to their host cells through posttranslational regulation of their host proteins. Finally, the ability of modified inteins to splice and cleave their fused exteins has enabled many new applications in protein science and synthetic biology. In this review, we briefly cover the mechanisms of protein splicing, evidence for some inteins as environmental sensors, and intein-based applications in protein engineering.

Neighboring inteins interfere with one another's homing capacity

Inteins are mobile genetic elements that invade conserved genes across all domains of life and viruses. In some instances, a single gene will have several intein insertion sites. In Haloarchaea, the minichromosome maintenance (MCM) protein at the core of replicative DNA helicase contains four intein insertion sites within close proximity, where two of these sites (MCM-a and MCM-d) are more likely to be invaded. A haloarchaeon that harbors both MCM-a and MCM-d inteins, Haloferax mediterranei, was studied in vivo to determine intein invasion dynamics and the interactions between neighboring inteins. Additionally, invasion frequencies and the conservation of insertion site sequences in 129 Haloferacales mcm homologs were analyzed to assess intein distribution across the order. We show that the inteins at MCM-a and MCM-d recognize and cleave their respective target sites and, in the event that only one empty intein invasion site is present, readily initiate homing (i.e. single homing). However, when two inteins are present co-homing into an intein-free target sequence is much less effective. The two inteins are more effective when invading alleles that already contain an intein at one of the two sites. Our in vivo and computational studies also support that having a proline in place of a serine as the first C-terminal extein residue of the MCM-d insertion site prevents successful intein splicing, but does not stop recognition of the insertion site by the intein's homing endonuclease.

Proteomic survey of the DNA damage response in Caulobacter crescentus

The bacterial DNA damage response is a critical, coordinated response to endogenous and exogenous sources of DNA damage. Response dynamics are dependent on coordinated synthesis and loss of relevant proteins. While much is known about its global transcriptional control, changes in protein abundance that occur upon DNA damage are less well characterized at the system level. Here, we perform a proteome-wide survey of the DNA damage response in Caulobacter crescentus . We find that while most protein abundance changes upon DNA damage are readily explained by changes in transcription, there are exceptions. The survey also allowed us to identify the novel DNA damage response factor, YaaA, which has been overlooked by previously published, transcription- focused studies. A similar survey in a Δ lon strain was performed to explore lon's role in DNA damage survival. The Δ lon strain had a smaller dynamic range of protein abundance changes in general upon DNA damage compared to the wild type strain. This system-wide change to the dynamics of the response may explain this strain's sensitivity to DNA damage. Our proteome survey of the DNA damage response provides additional insight into the complex regulation of stress response and nominates a novel response factor that was overlooked in prior studies.

IMPORTANCE

The DNA damage response helps bacteria to react to and potentially survive DNA damage. The mutagenesis induced during this stress response contributes to the development of antibiotic resistance. Understanding how bacteria coordinate their response to DNA damage could help us to combat this growing threat to human health. While the transcriptional regulation of the bacterial DNA damage response has been characterized, this study is the first to our knowledge to assess the proteomic response to DNA damage in Caulobacter .

Cryptococcus neoformans Prp8 Intein: An In Vivo Target-Based Drug Screening System in Saccharomyces cerevisiae to Identify Protein Splicing Inhibitors and Explore Its Dynamics

Inteins are genetic mobile elements that are inserted within protein-coding genes, which are usually housekeeping genes. They are transcribed and translated along with the host gene, then catalyze their own splicing out of the host protein, which assumes its functional conformation thereafter. As Prp8 inteins are found in several important fungal pathogens and are absent in mammals, they are considered potential therapeutic targets since inhibiting their splicing would selectively block the maturation of fungal proteins. We developed a target-based drug screening system to evaluate the splicing of Prp8 intein from the yeast pathogen Cryptococcus neoformans (CnePrp8i) using Saccharomyces cerevisiae Ura3 as a non-native host protein. In our heterologous system, intein splicing preserved the full functionality of Ura3. To validate the system for drug screening, we examined cisplatin, which has been described as an intein splicing inhibitor. By using our system, new potential protein splicing inhibitors may be identified and used, in the future, as a new class of drugs for mycosis treatment. Our system also greatly facilitates the visualization of CnePrp8i splicing dynamics in vivo.

SufB intein splicing in Mycobacterium tuberculosis is influenced by two remote conserved N-extein histidines

Inteins are auto-processing domains that implement a multistep biochemical reaction termed protein splicing, marked by cleavage and formation of peptide bonds. They excise from a precursor protein, generating a functional protein via covalent bonding of flanking exteins. We report the kinetic study of splicing and cleavage reaction in [Fe–S] cluster assembly protein SufB from Mycobacterium tuberculosis (Mtu). Although it follows a canonical intein splicing pathway, distinct features are added by extein residues present in the active site. Sequence analysis identified two conserved histidines in the N-extein region; His-5 and His-38. Kinetic analyses of His-5Ala and His-38Ala SufB mutants exhibited significant reductions in splicing and cleavage rates relative to the SufB wildtype (WT) precursor protein. Structural analysis and molecular dynamics (MD) simulations suggested that Mtu SufB displays a unique mechanism where two remote histidines work concurrently to facilitate N-terminal cleavage reaction. His-38 is stabilized by the solvent-exposed His-5, and can impact N–S acyl shift by direct interaction with the catalytic Cys1. Development of inteins as biotechnological tools or as pathogen-specific novel antimicrobial targets requires a more complete understanding of such unexpected roles of conserved extein residues in protein splicing.

Conditional Alternative Protein Splicing Promoted by Inteins from Haloquadratum walsbyi

Protein splicing is a post-translational process by which an intervening protein, or an intein, catalyzes its own excision from flanking polypeptides, or exteins, coupled to extein ligation. Four inteins interrupt the MCM helicase of the halophile Haloquadratum walsbyi, two of which are mini-inteins that lack a homing endonuclease. Both inteins can be overexpressed in Escherichia coli and purified as unspliced precursors; splicing can be induced in vitro by incubation with salt. However, one intein can splice in 0.5 M NaCl in vitro, whereas the other splices efficiently only in buffer containing over 2 M NaCl; the organism also requires high salt to grow, with the standard growth media containing over 3 M NaCl and about 0.75 M magnesium salts. Consistent with this difference in salt-dependent activity, an intein-containing precursor protein with both inteins promotes conditional alternative protein splicing (CAPS) to yield different spliced products dependent on the salt concentration. Native Trp fluorescence of the inteins suggests that the difference in activity may be due to partial unfolding of the inteins at lower salt concentrations. This differential salt sensitivity of intein activity may provide a useful mechanism for halophiles to respond to environmental changes.

Reactive Chlorine Species Reversibly Inhibit DnaB Protein Splicing in Mycobacteria

Intervening proteins, or inteins, are mobile genetic elements that are translated within host polypeptides and removed at the protein level by splicing. In protein splicing, a self-mediated reaction removes the intein, leaving a peptide bond in place. While protein splicing can proceed in the absence of external cofactors, several examples of conditional protein splicing (CPS) have emerged. In CPS, the rate and accuracy of splicing are highly dependent on environmental conditions. Because the activity of the intein-containing host protein is compromised prior to splicing and inteins are highly abundant in the microbial world, CPS represents an emerging form of posttranslational regulation that is potentially widespread in microbes. Reactive chlorine species (RCS) are highly potent oxidants encountered by bacteria in a variety of natural environments, including within cells of the mammalian innate immune system. Here, we demonstrate that two naturally occurring RCS, namely, hypochlorous acid (the active compound in bleach) and N-chlorotaurine, can reversibly block splicing of DnaB inteins from Mycobacterium leprae and Mycobacterium smegmatis in vitro. Further, using a reporter that monitors DnaB intein activity within M. smegmatis, we show that DnaB protein splicing is inhibited by RCS in the native host. DnaB, an essential replicative helicase, is the most common intein-housing protein in bacteria. These results add to the growing list of environmental conditions that are relevant to the survival of the intein-containing host and influence protein splicing, as well as suggesting a novel mycobacterial response to RCS. We propose a model in which DnaB splicing, and therefore replication, is paused when these mycobacteria encounter RCS. IMPORTANCE Inteins are both widespread and abundant in microbes, including within several bacterial and fungal pathogens. Inteins are domains translated within host proteins and removed at the protein level by splicing. Traditionally considered molecular parasites, some inteins have emerged in recent years as adaptive posttranslational regulatory elements. Several studies have demonstrated CPS, in which the rate and accuracy of protein splicing, and thus host protein functions, are responsive to environmental conditions relevant to the intein-containing organism. In this work, we demonstrate that two naturally occurring RCS, including the active compound in household bleach, reversibly inhibit protein splicing of Mycobacterium leprae and Mycobacterium smegmatis DnaB inteins. In addition to describing a new physiologically relevant condition that can temporarily inhibit protein splicing, this study suggests a novel stress response in Mycobacterium, a bacterial genus of tremendous importance to humans.

Intein Inhibitors as Novel Antimicrobials: Protein Splicing in Human Pathogens, Screening Methods, and Off-Target Considerations

Protein splicing is a post-translational process by which an intervening polypeptide, or intein, catalyzes its own removal from the flanking polypeptides, or exteins, concomitant with extein ligation. Although inteins are highly abundant in the microbial world, including within several human pathogens, they are absent in the genomes of metazoans. As protein splicing is required to permit function of essential proteins within pathogens, inteins represent attractive antimicrobial targets. Here we review key proteins interrupted by inteins in pathogenic mycobacteria and fungi, exciting discoveries that provide proof of concept that intein activity can be inhibited and that this inhibition has an effect on the host organism's fitness, and bioanalytical methods that have been used to screen for intein activity. We also consider potential off-target inhibition of hedgehog signaling, given the similarity in structure and function of inteins and hedgehog autoprocessing domains.

Monitoring Protein Splicing Using In-gel Fluorescence Immediately Following SDS-PAGE

Inteins garner significant interest from both basic and applied researchers due to their unique catalytic abilities. Herein, we describe a protocol for accurately monitoring protein splicing without purification using in-gel fluorescence immediately following Tris-Glycine SDS-PAGE. Following expression in Escherichia coli, cells are lysed by sonication, cell supernatants are separated using Tris-Glycine SDS-PAGE, and superfolder GFP (sfGFP) fluorescence is directly visualized within gels. This method is rapid, with sfGFP immediately imaged following SDS-PAGE without staining. Further, sfGFP can be specifically detected in complex samples such as E. coli cell supernatants, proteins run at expected masses, and all steps can be performed at ambient temperature. This strategy is broadly applicable beyond the study of protein splicing and can be used for sensitive and specific visualization of superfolder sfGFP-tagged proteins in-gel.

Mini-Intein Structures from Extremophiles Suggest a Strategy for Finding Novel Robust Inteins

Inteins are prevalent among extremophiles. Mini-inteins with robust splicing properties are of particular interest for biotechnological applications due to their small size. However, biochemical and structural characterization has still been limited to a small number of inteins, and only a few serve as widely used tools in protein engineering. We determined the crystal structure of a naturally occurring Pol-II mini-intein from Pyrococcus horikoshii and compared all three mini-inteins found in the genome of P. horikoshii. Despite their similar sizes, the comparison revealed distinct differences in the insertions and deletions, implying specific evolutionary pathways from distinct ancestral origins. Our studies suggest that sporadically distributed mini-inteins might be more promising for further protein engineering applications than highly conserved mini-inteins. Structural investigations of additional inteins could guide the shortest path to finding novel robust mini-inteins suitable for various protein engineering purposes.

Construction and Quantitation of a Selectable Protein Splicing Sensor Using Gibson Assembly and Spot Titers

Inteins (intervening proteins) are translated within host proteins and removed through protein splicing. Conditional protein splicing (CPS), where the rate and accuracy of splicing are highly dependent on environmental cues, has emerged as a novel form of post-translational regulation. While CPS has been demonstrated for several inteins in vitro, a comprehensive understanding of inteins requires tools to quantitatively monitor their activity within the cellular context. Here, we describe a method for construction of a splicing-dependent system that can be used to quantitatively assay for conditions that modulate protein splicing. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Construction of an intein-containing KanR2 library using Gibson assembly Basic Protocol 2: Phenotype determination using quantitative spot titers Support Protocol 1: Preparation of LB agar plates for spot titers Support Protocol 2: Preparation and transformation of competent M. smegmatis cells.

Metal effect on intein splicing: A review

Inteins are intervening polypeptides that interrupt the functional domains of several important proteins across the three domains of life. Inteins excise themselves from the precursor protein, ligating concomitant extein residues in a process called protein splicing. Post-translational auto-removal of inteins remain critical for the generation of active proteins. The perspective of inteins in science is a robust field of research, however fundamental studies centralized upon splicing regulatory mechanism are imperative for addressing more intricate issues. Controlled engineering of intein splicing has many applications; intein inhibition can facilitate novel drug design, while activation of intein splicing is exploited in protein purification. This paper provides a comprehensive review of the past and recent advances in the splicing regulation via metal-intein interaction. We compare the behavior of different metal ions on diverse intein systems. Though metals such as Zn, Cu, Pt, Cd, Co, Ni exhibit intein inhibitory effect heterogeneously on different inteins, divalent metal ions such as Ca and Mg fail to do so. The observed diversity in the metal-intein interaction arises mostly due to intein polymorphism and variations in atomic structure of metals. A mechanistic understanding of intein regulation by metals in native as well as synthetically engineered intein systems may yield potent intein inhibitors via direct or indirect approach.

Structural basis of ubiquitination mediated by protein splicing in early Eukarya

BACKGROUND

Inteins are intervening proteins, which are known to perform protein splicing. The reaction results in the production of an intein domain and an inteinless protein, which shows no trace of the insertion. BIL2 is part of the polyubiquitin locus of Tetrahymena thermophila (BUBL), where two bacterial-intein-like (BIL) domains lacking the C + 1 nucleophile, are flanked by two independent ubiquitin-like domains (ubl4/ubl5).

METHODS

We solved the X-ray structures of BIL2 in both the inactive and unprecedented, zinc-induced active, forms. Then, we characterized by mass spectrometry the BUBL splicing products in the absence and in the presence of T.thRas-GTPase. Finally, we investigated the effect of ubiquitination on T.thRas-GTPase by molecular dynamics simulations.

RESULTS

The structural analysis demonstrated that zinc-induced conformational change activates protein splicing. Moreover, mass spectrometry characterization of the splicing products shed light on the possible function of BIL2, which operates as a "single-ubiquitin-dispensing-platform", allowing the conjugation, via isopeptide bond formation (K(εNH₂)-C-ter), of ubl4 to either ubl5 or T.thRas-GTPase. Lastly, we demonstrated that T.thRas-GTPase ubiquitination occurs in proximity of the nucleotide binding pocket and stabilizes the protein active state.

CONCLUSIONS

We demonstrated that BIL2 is activated by zinc and that protein splicing induced by this intein does not take place through classical or aminolysis mechanisms but via formation of a covalent isopeptide bond, causing the ubiquitination of endogenous substrates such as T.thRas-GTPase.

GENERAL SIGNIFICANCE

In this "enzyme-free" ubiquitination mechanism the isopeptide formation, which canonically requires E1-E2-E3 enzymatic cascade and constitutes the alphabet of ubiquitin biology, is achieved in a single, concerted step without energy consumption.

Inteins in Science: Evolution to Application

Inteins are mobile genetic elements that apply standard enzymatic strategies to excise themselves post-translationally from the precursor protein via protein splicing. Since their discovery in the 1990s, recent advances in intein technology allow for them to be implemented as a modern biotechnological contrivance. Radical improvement in the structure and catalytic framework of cis- and trans-splicing inteins devised the development of engineered inteins that contribute to various efficient downstream techniques. Previous literature indicates that implementation of intein-mediated splicing has been extended to in vivo systems. Besides, the homing endonuclease domain also acts as a versatile biotechnological tool involving genetic manipulation and control of monogenic diseases. This review orients the understanding of inteins by sequentially studying the distribution and evolution pattern of intein, thereby highlighting a role in genetic mobility. Further, we include an in-depth summary of specific applications branching from protein purification using self-cleaving tags to protein modification, post-translational processing and labelling, followed by the development of intein-based biosensors. These engineered inteins offer a disruptive approach towards research avenues like biomaterial construction, metabolic engineering and synthetic biology. Therefore, this linear perspective allows for a more comprehensive understanding of intein function and its diverse applications.

Protein Splicing Activity of the Haloferax volcanii PolB-c Intein Is Sensitive to Homing Endonuclease Domain Mutations

Inteins are selfish genetic elements residing in open reading frames that can splice post-translationally, resulting in the ligation of an uninterrupted, functional protein. Like other inteins, the DNA polymerase B (PolB) intein of the halophilic archaeon Haloferax volcanii has an active homing endonuclease (HEN) domain, facilitating its horizontal transmission. Previous work has shown that the presence of the PolB intein exerts a significant fitness cost on the organism compared to an intein-free isogenic H. volcanii. Here, we show that mutation of a conserved residue in the HEN domain not only reduces intein homing but also slows growth. Surprisingly, although this mutation is far from the protein splicing active site, it also significantly reduces in vitro protein splicing. Moreover, two additional HEN domain mutations, which could not be introduced to H. volcanii, presumably due to lethality, also eliminate protein splicing activity in vitro. These results suggest an interplay between HEN residues and the protein splicing domain, despite an over 35 Å separation in a PolB intein homology model. The combination of in vivo and in vitro evidence strongly supports a model of codependence between the self-splicing domain and the HEN domain that has been alluded to by previous in vitro studies of protein splicing with HEN domain-containing inteins.

Conditional DnaB Protein Splicing Is Reversibly Inhibited by Zinc in Mycobacteria

Inteins, as posttranslational regulatory elements, can tune protein function to environmental changes by conditional protein splicing (CPS). Translated as subdomains interrupting host proteins, inteins splice to scarlessly join flanking sequences (exteins). We used DnaB-intein1 (DnaBi1) from a replicative helicase of Mycobacterium smegmatis to build a kanamycin intein splicing reporter (KISR) that links splicing of DnaBi1 to kanamycin resistance. Using expression in heterologous Escherichia coli, we observed phenotypic classes of various levels of splicing-dependent resistance (SDR) and related these to the insertion position of DnaBi1 within the kanamycin resistance protein (KanR). The KanR-DnaBi1 construct demonstrating the most stringent SDR was used to probe for CPS of DnaB in the native host environment, M. smegmatis We show here that zinc, important during mycobacterial pathogenesis, inhibits DnaB splicing in M. smegmatis Using an in vitro reporter system, we demonstrated that zinc potently and reversibly inhibited DnaBi1 splicing, as well as splicing of a comparable intein from Mycobacterium leprae Finally, in a 1.95 Å crystal structure, we show that zinc inhibits splicing through binding to the very cysteine that initiates the splicing reaction. Together, our results provide compelling support for a model whereby mycobacterial DnaB protein splicing, and thus DNA replication, is responsive to environmental zinc.IMPORTANCE Inteins are present in a large fraction of prokaryotes and localize within conserved proteins, including the mycobacterial replicative helicase DnaB. In addition to their extensive protein engineering applications, inteins have emerged as environmentally responsive posttranslational regulators of the genes that encode them. While several studies have shown compelling evidence of conditional protein splicing (CPS), examination of splicing in the native host of the intein has proven to be challenging. Here, we demonstrated through a number of measures, including the use of a splicing-dependent sensor capable of monitoring intein activity in the native host, that zinc is a potent and reversible inhibitor of mycobacterial DnaB splicing. This work also expands our knowledge of site selection for intein insertion within nonnative proteins, demonstrating that splicing-dependent host protein activation correlates with proximity to the active site. Additionally, we surmise that splicing regulation by zinc has mycobacteriocidal and CPS application potential.

Allosteric Influence of Extremophile Hairpin Motif Mutations on the Protein Splicing Activity of a Hyperthermophilic Intein

Protein splicing is a post-translational process mediated by an intein, whereby the intein excises itself from a precursor protein with concomitant ligation of the two flanking polypeptides. The intein that interrupts the DNA polymerase II in the extreme hyperthermophile Pyrococcus abyssi has a β-hairpin that extends the central β-sheet of the intein. This β-hairpin is mostly found in inteins from archaea, as well as halophilic eubacteria, and is thus called the extremophile hairpin (EXH) motif. The EXH is stabilized by multiple favorable interactions, including electrostatic interactions involving Glu29, Glu31, and Arg40. Mutations of these residues diminish the extent of N-terminal cleavage and the extent of protein splicing, likely by interfering with the coordination of the steps of splicing. These same mutations decrease the global stability of the intein fold as measured by susceptibility to thermolysin cleavage. ¹⁵N-¹H heteronuclear single-quantum coherence demonstrated that these mutations altered the chemical environment of active site residues such as His93 (B-block histidine) and Ser166 (F-block residue 4). This work again underscores the connected and coordinated nature of intein conformation and dynamics, where remote mutations can disturb a finely tuned interaction network to inhibit or enhance protein splicing.

Genomic Analysis of Mic1 Reveals a Novel Freshwater Long-Tailed Cyanophage

Lake Chaohu, one of the five largest freshwater lakes in China, has been suffering from severe cyanobacterial blooms in the summer for many years. Cyanophages, the viruses that specifically infect cyanobacteria, play a key role in modulating cyanobacterial population, and thus regulate the emergence and decline of cyanobacterial blooms. Here we report a long-tailed cyanophage isolated from Lake Chaohu, termed Mic1, which specifically infects the cyanobacterium Microcystis aeruginosa. Mic1 has an icosahedral head of 88 nm in diameter and a long flexible tail of 400 nm. It possesses a circular genome of 92,627 bp, which contains 98 putative open reading frames. Genome sequence analysis enabled us to define a novel terminase large subunit that consists of two types of intein, indicating that the genome packaging of Mic1 is under fine control via posttranslational maturation of the terminase. Moreover, phylogenetic analysis suggested Mic1 and mitochondria share a common evolutionary origin of DNA polymerase γ gene. All together, these findings provided a start-point for investigating the co-evolution of cyanophages and its cyanobacterial hosts.

Cyanobacterial multi-copy chromosomes and their replication

While the model bacteria Escherichia coli and Bacillus subtilis harbor single chromosomes, which is known as monoploidy, some freshwater cyanobacteria contain multiple chromosome copies per cell throughout their cell cycle, which is known as polyploidy. In the model cyanobacteria Synechococcus elongatus PCC 7942 and Synechocystis sp. PCC 6803, chromosome copy number (ploidy) is regulated in response to growth phase and environmental factors. In S. elongatus 7942, chromosome replication is asynchronous both among cells and chromosomes. Comparative analysis of S. elongatus 7942 and S. sp. 6803 revealed a variety of DNA replication mechanisms. In this review, the current knowledge of ploidy and DNA replication mechanisms in cyanobacteria is summarized together with information on the features common with plant chloroplasts. It is worth noting that the occurrence of polyploidy and its regulation are correlated with certain cyanobacterial lifestyles and are shared between some cyanobacteria and chloroplasts.

ABBREVIATIONS

NGS: next-generation sequencing; Repli-seq: replication sequencing; BrdU: 5-bromo-2'-deoxyuridine; TK: thymidine kinase; GCSI: GC skew index; PET: photosynthetic electron transport; RET: respiration electron transport; Cyt b₆f complex: cytochrome b₆f complex; PQ: plastoquinone; PC: plastocyanin.

The Mechanism of Cholesterol Modification of Hedgehog Ligand

Hedgehog (Hh) proteins are important components of signal transduction pathways involved in animal development, and their defects are implicated in carcinogenesis. Their N-terminal domain (HhN) acts as a signaling ligand, and their C-terminal domain (HhC) performs an autocatalytic function of cleaving itself away, while adding a cholesterol moiety to HhN. HhC has two sub-domains: a hedgehog/intein (hint) domain that primarily performs the autocatalytic activity, and a sterol-recognition region (SRR) that binds to cholesterol and properly positions it with respect to HhN. The three-dimensional details of this autocatalytic mechanism remain unknown, as does the structure of the precursor Hh protein. In this study, a complete cholesterol-bound precursor form of the drosophila Hh precursor is modeled using known crystal structures of HhN and the hint domain, and a hypothesized similarity of SRR to an unrelated but similar-sized cholesterol binding protein. The restrained geometries and topology switching (RGATS) strategy is then used to predict atomic-detail pathways for the full autocatalytic reaction starting from the precursor and ending in a cholesterol-linked HhN domain and a cleaved HhC domain. The RGATS explicit solvent simulations indicate the roles of individual HhC residues in facilitating the reaction, which can be confirmed through mutational experiments. These simulations also provide plausible structural models for the N/S acyl transfer intermediate and the product states of this reaction. This study thus provides a good framework for future computational and experimental studies to develop a full structural and dynamic understanding of Hh autoprocessing. © 2019 Wiley Periodicals, Inc.

PSSP: Protein splice site prediction algorithm using Bayesian approach

This study aimed to introduce an algorithm and identify intein motif and blocks involved in protein splicing, and explore the underlying methods in the development of detection of protein motifs. Inteins are mobile protein splicing elements capable of self-splicing post-translationally. They exist in viruses and bacteriophage, notwithstanding this broad phylogenetic distribution, all inteins apportion common structural features. A method was developed to predict intein in a raw sequence, using a ranking and scoring scheme based on amino acid θ value tables. This method aided in the identification and assessment of patterns characterizing the intein sequences. New intein conserved properties are revealed and the known ones are described and localized. We have computed the θ value of each amino acid at block A positions +1 to +13, block B positions l+13 to l+26 and block G positions -7 to +1 for the three categories. The consensus amino acids thus found are listed at the end of each row. We gave statistics for the distance between the blocks, block A to B, block B to F, and block F to G with the average being 66.1, 294, and 10.2 amino acids, respectively. The actual blocks A, B, and G of the one intein found in vacuolar membrane ATPase subunit, a precursor protein, are ranked 1. The results indicate all of the block sequences that are found in nine proteins are ranked at top of the list. The intein sequence is used to search the databases for intein-like proteins. Understanding the functional, structural, and dynamical aspects of inteins is important for intein engineering and the betterment of intein database.

Efficient Generation of Hydrazides in Proteins by RadA Split Intein

Protein C-terminal hydrazides are useful for bioconjugation and construction of proteins from multiple fragments through native chemical ligation. To generate C-terminal hydrazides in proteins, an efficient intein-based preparation method has been developed by using thiols and hydrazine to accelerate the formation of the transient thioester intermediate and subsequent hydrazinolysis. This approach not only increases the yield, but also improves biocompatibility. The scope of the method has been expanded by employing Pyrococcus horikoshii RadA split intein, which can accommodate a broad range of extein residues before the site of cleavage. The use of split RadA minimizes premature intein N cleavage in vivo and offers control over the initiation of the intein N cleavage reaction. It is expected that this versatile preparation method will expand the utilization of protein C-terminal hydrazides in protein preparation and modification.

Identification, Characterization, and Optimization of Split Inteins

In recent years, split inteins have seen widespread use as molecular platforms for the design of a variety of peptide and protein chemistry technologies, most notably protein ligation. The development of these approaches is dependent on the identification and/or design of split inteins with robust activity, stability, and solubility. Here, we describe two approaches to characterize and compare the activities of newly identified or engineered split inteins. The first assay employs an E. coli-based selection system to rapidly screen the activities of many inteins and can be repurposed for directed evolution. The second assay utilizes reverse-phase high-performance liquid chromatography (RP-HPLC) to provide insights into individual chemical steps in the protein splicing reaction, information that can guide further engineering efforts. These techniques provide useful alternatives to common assays that utilize SDS-PAGE to analyze splicing reaction progress.

Spliceosomal Prp8 intein at the crossroads of protein and RNA splicing

The spliceosome is a large ribonucleoprotein complex that removes introns from pre-mRNAs. At its functional core lies the essential pre-mRNA processing factor 8 (Prp8) protein. Across diverse eukaryotes, this protein cofactor of RNA catalysis harbors a self-splicing element called an intein. Inteins in Prp8 are extremely pervasive and are found at 7 different sites in various species. Here, we focus on the Prp8 intein from Cryptococcus neoformans (Cne), a human fungal pathogen. We solved the crystal structure of this intein, revealing structural homology among protein splicing sequences in eukaryotes, including the Hedgehog C terminus. Working with the Cne Prp8 intein in a reporter assay, we find that the biologically relevant divalent metals copper and zinc inhibit intein splicing, albeit by 2 different mechanisms. Copper likely stimulates reversible modifications on a catalytically important cysteine, whereas zinc binds at the terminal asparagine and the same critical cysteine. Importantly, we also show that copper treatment inhibits Prp8 protein splicing in Cne. Lastly, an intein-containing Prp8 precursor model is presented, suggesting that metal-induced protein splicing inhibition would disturb function of both Prp8 and the spliceosome. These results indicate that Prp8 protein splicing can be modulated, with potential functional implications for the spliceosome.

Crystal structures of CDC21-1 inteins from hyperthermophilic archaea reveal the selection mechanism for the highly conserved homing endonuclease insertion site

Self-splicing inteins are mobile genetic elements invading host genes via nested homing endonuclease (HEN) domains. All HEN domains residing within inteins are inserted at a highly conserved insertion site. A purifying selection mechanism directing the location of the HEN insertion site has not yet been identified. In this work, we solved the three-dimensional crystal structures of two inteins inserted in the cell division control protein 21 of the hyperthermophilic archaea Pyrococcus abyssi and Pyrococcus horikoshii. A comparison between the structures provides the structural basis for the thermo-stabilization mechanism of inteins that have lost the HEN domain during evolution. The presence of an entire extein domain in the intein structure from Pyrococcus horikoshii suggests the selection mechanism for the highly conserved HEN insertion point.

Mechanism of Single-Stranded DNA Activation of Recombinase Intein Splicing

Inteins, or intervening proteins, are mobile genetic elements translated within host polypeptides and removed through protein splicing. This self-catalyzed process breaks two peptide bonds and rejoins the flanking sequences, called N- and C-exteins, with the intein scarlessly escaping the host protein. As these elements have traditionally been viewed as purely selfish genetic elements, recent work has demonstrated that the conditional protein splicing (CPS) of several naturally occurring inteins can be regulated by a variety of environmental cues relevant to the survival of the host organism or crucial to the invading protein function. The RadA recombinase from the archaeon Pyrococcus horikoshii represents an intriguing example of CPS, whereby protein splicing is inhibited by interactions between the intein and host protein C-extein. Single-stranded DNA (ssDNA), a natural substrate of RadA as well as signal that recombinase activity is needed by the cell, dramatically improves the splicing rate and accuracy. Here, we investigate the mechanism by which ssDNA exhibits this influence and find that ssDNA strongly promotes a specific step of the splicing reaction, cyclization of the terminal asparagine of the intein. Interestingly, inhibitory interactions between the host protein and intein that block splicing localize to this asparagine, suggesting that ssDNA binding alleviates this inhibition to promote splicing. We also find that ssDNA directly influences the position of catalytic nucleophiles required for protein splicing, implying that ssDNA promotes assembly of the intein active site. This work advances our understanding of how ssDNA accelerates RadA splicing, providing important insights into this intriguing example of CPS.

PRP8 Intein in Onygenales: Distribution and Phylogenetic Aspects

Inteins (internal proteins) are mobile genetic elements, inserted in housekeeping proteins, with self-splicing properties. Some of these elements have been recently pointed out as modulators of genetic expression or protein function. Herein, we evaluated, in silico, the distribution and phylogenetic patterns of PRP8 intein among 93 fungal strains of the order Onygenales. PRP8 intein(s) are present in most of the species (45/49), mainly as full-length inteins (containing both the Splicing and the Homing Endonuclease domains), and must have transferred vertically in all lineages, since their phylogeny reflects the group phylogeny. While the distribution of PRP8 intein(s) varies among species of Onygenaceae family, being absent in Coccidioides spp. and present as full and mini-intein in other species, they are consistently observed as full-length inteins in all evaluated pathogenic species of the Arthrodermataceae and Ajellomycetaceae families. This conservative and massive PRP8 intein presence in Ajellomycetacean and Arthrodermatecean species reinforces the previous idea that such genetic elements do not decrease the fungal fitness significantly and even might play some role in the host-pathogen relationship, at least in these two fungal groups. We may better position the species Ophidiomyces ophiodiicola (with no intein) in the Onygenaceae family and Onygena corvina (with a full-length intein) as a basal member in the Arthrodermataceae family.

Biotechnology of extremely thermophilic archaea

Although the extremely thermophilic archaea (Topt ≥ 70°C) may be the most primitive extant forms of life, they have been studied to a limited extent relative to mesophilic microorganisms. Many of these organisms have unique biochemical and physiological characteristics with important biotechnological implications. These include methanogens that generate methane, fermentative anaerobes that produce hydrogen gas with high efficiency, and acidophiles that can mobilize base, precious and strategic metals from mineral ores. Extremely thermophilic archaea have also been a valuable source of thermoactive, thermostable biocatalysts, but their use as cellular systems has been limited because of the general lack of facile genetics tools. This situation has changed recently, however, thereby providing an important avenue for understanding their metabolic and physiological details and also opening up opportunities for metabolic engineering efforts. Along these lines, extremely thermophilic archaea have recently been engineered to produce a variety of alcohols and industrial chemicals, in some cases incorporating CO2 into the final product. There are barriers and challenges to these organisms reaching their full potential as industrial microorganisms but, if these can be overcome, a new dimension for biotechnology will be forthcoming that strategically exploits biology at high temperatures.

Mycobacterial DnaB helicase intein as oxidative stress sensor

Inteins are widespread self-splicing protein elements emerging as potential post-translational environmental sensors. Here, we describe two inteins within one protein, the Mycobacterium smegmatis replicative helicase DnaB. These inteins, DnaBi1 and DnaBi2, have homology to inteins in pathogens, splice with vastly varied rates, and are differentially responsive to environmental stressors. Whereas DnaBi1 splicing is reversibly inhibited by oxidative and nitrosative insults, DnaBi2 is not. Using a reporter that measures splicing in a native intein-containing organism and western blotting, we show that H2O2 inhibits DnaBi1 splicing in M. smegmatis. Intriguingly, upon oxidation, the catalytic cysteine of DnaBi1 forms an intramolecular disulfide bond. We report a crystal structure of the class 3 DnaBi1 intein at 1.95 Å, supporting our findings and providing insight into this splicing mechanism. We propose that this cysteine toggle allows DnaBi1 to sense stress, pausing replication to maintain genome integrity, and then allowing splicing immediately when permissive conditions return.

Inteins: Localized Distribution, Gene Regulation, and Protein Engineering for Biological Applications

Inteins are self-splicing polypeptides with an ability to excise themselves from flanking host protein regions with remarkable precision; in the process, they ligate flanked host protein fragments. Inteins are distributed sporadically across all three domains of life (bacteria, archaea, and unicellular eukaryotes). However, their apparent localized distribution in DNA replication, repair, and recombination proteins (the 3Rs), particularly in bacteria and archaea, is enigmatic. Our understanding of the localized distribution of inteins in the 3Rs, and their possible regulatory role in such distribution, is still only partial. Nevertheless, understanding the chemistry of post-translational self-splicing of inteins has opened up opportunities for protein chemists to modify, manipulate, and bioengineer proteins. Protein-splicing technology is adapted to a wide range of applications, starting with untagged protein purification, site-specific protein labeling, protein biotinylation, isotope incorporation, peptide cyclization, as an antimicrobial target, and so on. This review is focused on the chemistry of splicing; the localized distribution of inteins, particularly in the 3Rs and their possible role in regulating host protein function; and finally, the use of protein-splicing technology in various protein engineering applications.

Conditional Protein Splicing Switch in Hyperthermophiles through an Intein-Extein Partnership

Inteins are intervening proteins that undergo an autocatalytic splicing reaction that ligates flanking host protein sequences termed exteins. Some intein-containing proteins have evolved to couple splicing to environmental signals; this represents a new form of posttranslational regulation. Of particular interest is RadA from the archaeon Pyrococcus horikoshii, for which long-range intein-extein interactions block splicing, requiring temperature and single-stranded DNA (ssDNA) substrate to splice rapidly and accurately. Here, we report that splicing of the intein-containing RadA from another archaeon, Thermococcus sibericus, is activated by significantly lower temperatures than is P. horikoshii RadA, consistent with differences in their growth environments. Investigation into variations between T. sibericus and P. horikoshii RadA inteins led to the discovery that a nonconserved region (NCR) of the intein, a flexible loop where a homing endonuclease previously resided, is critical to splicing. Deletion of the NCR leads to a substantial loss in the rate and accuracy of P. horikoshii RadA splicing only within native exteins. The influence of the NCR deletion can be largely overcome by ssDNA, demonstrating that the splicing-competent conformation can be achieved. We present a model whereby the NCR is a flexible hinge which acts as a switch by controlling distant intein-extein interactions that inhibit active site assembly. These results speak to the repurposing of the vestigial endonuclease loop to control an intein-extein partnership, which ultimately allows exquisite adaptation of protein splicing upon changes in the environment.

Inteins are mobile genetic elements that interrupt coding sequences (exteins) and are removed by protein splicing. They are abundant elements in microbes, and recent work has demonstrated that protein splicing can be controlled by environmental cues, including the substrate of the intein-containing protein. Here, we describe an intein-extein collaboration that controls temperature-induced splicing of RadA from two archaea and how variation in this intein-extein partnership results in fine-tuning of splicing to closely match the environment. Specifically, we found that a small sequence difference between the two inteins, a flexible loop that likely once housed a homing endonuclease used for intein mobility, acts as a switch to control intein-extein interactions that block splicing. Our results argue strongly that some inteins have evolved away from a purely parasitic lifestyle to control the activity of host proteins, representing a new form of posttranslational regulation that is potentially widespread in the microbial world.

The dynamic intein landscape of eukaryotes

BACKGROUND

Inteins are mobile, self-splicing sequences that interrupt proteins and occur across all three domains of life. Scrutiny of the intein landscape in prokaryotes led to the hypothesis that some inteins are functionally important. Our focus shifts to eukaryotic inteins to assess their diversity, distribution, and dissemination, with the aim to comprehensively evaluate the eukaryotic intein landscape, understand intein maintenance, and dissect evolutionary relationships.

RESULTS

This bioinformatics study reveals that eukaryotic inteins are scarce, but present in nuclear genomes of fungi, chloroplast genomes of algae, and within some eukaryotic viruses. There is a preponderance of inteins in several fungal pathogens of humans and plants. Inteins are pervasive in certain proteins, including the nuclear RNA splicing factor, Prp8, and the chloroplast DNA helicase, DnaB. We find that eukaryotic inteins frequently localize to unstructured loops of the host protein, often at highly conserved sites. More broadly, a sequence similarity network analysis of all eukaryotic inteins uncovered several routes of intein mobility. Some eukaryotic inteins appear to have been acquired through horizontal transfer with dsDNA viruses, yet other inteins are spread through intragenomic transfer. Remarkably, endosymbiosis can explain patterns of DnaB intein inheritance across several algal phyla, a novel mechanism for intein acquisition and distribution.

CONCLUSIONS

Overall, an intriguing picture emerges for how the eukaryotic intein landscape arose, with many evolutionary forces having contributed to its current state. Our collective results provide a framework for exploring inteins as novel regulatory elements and innovative drug targets.

Structural Basis for the Persistence of Homing Endonucleases in Transcription Factor IIB Inteins

Inteins are mobile genetic elements that are spliced out of proteins after translation. Some inteins contain a homing endonuclease (HEN) responsible for their propagation. Hedgehog/INTein (HINT) domains catalyzing protein splicing and their nested HEN domains are thought to be functionally independent because of the existence of functional mini-inteins without HEN domains. Despite the lack of obvious mutualism between HEN and HINT domains, HEN domains are persistently found at one specific site in inteins, indicating their potential functional role in protein splicing. Here we report crystal structures of inactive and active mini-inteins derived from inteins residing in the transcription factor IIB of Methanococcus jannaschii and Methanocaldococcus vulcanius, revealing a novel modified HINT fold that might provide new insights into the mutualism between the HEN and HINT domains. We propose an evolutionary model of inteins and a functional role of HEN domains in inteins.

Mobile self-splicing introns and inteins as environmental sensors

Self-splicing introns and inteins are often mobile at the level of the genome. Although these RNA and protein elements, respectively, are generally considered to be selfish parasites, group I and group II introns and inteins can be triggered by environmental cues to splice and/or to mobilize. These cues include stressors such as oxidizing agents, reactive oxygen and nitrogen species, starvation, temperature, osmolarity and DNA damage. Their sensitivity to these stimuli leads to a carefully choreographed dance between the mobile element and its host that is in tune with the cellular environment. This responsiveness to a changing milieu provides strong evidence that these diverse, self-splicing mobile elements have adapted to react to prevailing conditions, to the potential advantage of both the element and its host.

Exploring the genomic diversity of black yeasts and relatives (Chaetothyriales, Ascomycota)

The order Chaetothyriales (Pezizomycotina, Ascomycetes) harbours obligatorily melanised fungi and includes numerous etiologic agents of chromoblastomycosis, phaeohyphomycosis and other diseases of vertebrate hosts. Diseases range from mild cutaneous to fatal cerebral or disseminated infections and affect humans and cold-blooded animals globally. In addition, Chaetothyriales comprise species with aquatic, rock-inhabiting, ant-associated, and mycoparasitic life-styles, as well as species that tolerate toxic compounds, suggesting a high degree of versatile extremotolerance. To understand their biology and divergent niche occupation, we sequenced and annotated a set of 23 genomes of main the human opportunists within the Chaetothyriales as well as related environmental species. Our analyses included fungi with diverse life-styles, namely opportunistic pathogens and closely related saprobes, to identify genomic adaptations related to pathogenesis. Furthermore, ecological preferences of Chaetothyriales were analysed, in conjuncture with the order-level phylogeny based on conserved ribosomal genes. General characteristics, phylogenomic relationships, transposable elements, sex-related genes, protein family evolution, genes related to protein degradation (MEROPS), carbohydrate-active enzymes (CAZymes), melanin synthesis and secondary metabolism were investigated and compared between species. Genome assemblies varied from 25.81 Mb (Capronia coronata) to 43.03 Mb (Cladophialophora immunda). The bantiana-clade contained the highest number of predicted genes (12 817 on average) as well as larger genomes. We found a low content of mobile elements, with DNA transposons from Tc1/Mariner superfamily being the most abundant across analysed species. Additionally, we identified a reduction of carbohydrate degrading enzymes, specifically many of the Glycosyl Hydrolase (GH) class, while most of the Pectin Lyase (PL) genes were lost in etiological agents of chromoblastomycosis and phaeohyphomycosis. An expansion was found in protein degrading peptidase enzyme families S12 (serine-type D-Ala-D-Ala carboxypeptidases) and M38 (isoaspartyl dipeptidases). Based on genomic information, a wide range of abilities of melanin biosynthesis was revealed; genes related to metabolically distinct DHN, DOPA and pyomelanin pathways were identified. The MAT (MAting Type) locus and other sex-related genes were recognized in all 23 black fungi. Members of the asexual genera Fonsecaea and Cladophialophora appear to be heterothallic with a single copy of either MAT-1-1 or MAT-1-2 in each individual. All Capronia species are homothallic as both MAT1-1 and MAT1-2 genes were found in each single genome. The genomic synteny of the MAT-locus flanking genes (SLA2-APN2-COX13) is not conserved in black fungi as is commonly observed in Eurotiomycetes, indicating a unique genomic context for MAT in those species. The heterokaryon (het) genes expansion associated with the low selective pressure at the MAT-locus suggests that a parasexual cycle may play an important role in generating diversity among those fungi.

Protein splicing of a recombinase intein induced by ssDNA and DNA damage

In this study, Lennon et al. provide new insights into the role of inteins (or protein introns), which are known to autocatalytically excise themselves through protein splicing. They show that intein splicing can be stimulated by a substrate of the invaded host protein, suggesting a new form of post-translational control.

Inteins (or protein introns) autocatalytically excise themselves through protein splicing. We challenge the long-considered notion that inteins are merely molecular parasites and posit that some inteins evolved to regulate host protein function. Here we show substrate-induced and DNA damage-induced splicing, in which an archaeal recombinase RadA intein splices dramatically faster and more accurately when provided with ssDNA. This unprecedented example of intein splicing stimulation by the substrate of the invaded host protein provides compelling support in favor of inteins acting as pause buttons to arrest protein function until needed; then, an immediate activity switch is triggered, representing a new form of post-translational control.

Evolution and Application of Inteins in Candida species: A Review

Inteins are invasive intervening sequences that perform an autocatalytic splicing from their host proteins. Among eukaryotes, these elements are present in many fungal species, including those considered opportunistic or primary pathogens, such as Candida spp. Here we reviewed and updated the list of Candida species containing inteins in the genes VMA, THRRS and GLT1 and pointed out the importance of these elements as molecular markers for molecular epidemiological researches and species-specific diagnosis, since the presence, as well as the size of these inteins, is polymorphic among the different species. Although absent in Candida albicans, these elements are present in different sizes, in some environmental Candida spp. and also in most of the non-albicans Candida spp. considered emergent opportunistic pathogens. Besides, the possible role of these inteins in yeast physiology was also discussed in the light of the recent findings on the importance of these elements as post-translational modulators of gene expression, reinforcing their relevance as alternative therapeutic targets for the treatment of non-albicans Candida infections, because, once the splicing of an intein is inhibited, its host protein, which is usually a housekeeping protein, becomes non-functional.

Salt-inducible Protein Splicing in cis and trans by Inteins from Extremely Halophilic Archaea as a Novel Protein-Engineering Tool

Intervening protein sequences (inteins) from extremely halophilic haloarchaea can be inactive under low salinity but could be activated by increasing the salt content to a specific concentration for each intein. The halo-obligatory inteins confer high solubility under both low and high salinity conditions. We showed the broad utility of salt-dependent protein splicing in cis and trans by demonstrating backbone cyclization, self-cleavage for purification, and scarless protein ligation for segmental isotopic labeling. Artificially split MCM2 intein derived from Halorhabdus utahensis remained highly soluble and was capable of protein trans-splicing with excellent ligation kinetics by reassembly under high salinity conditions. Importantly, the MCM2 intein has the active site residue of Ser at the +1 position, which remains in the ligated product, instead of Cys as found in many other efficient split inteins. Since Ser is more abundant than Cys in proteins, the novel split intein could widen the applications of segmental labeling in protein NMR spectroscopy and traceless protein ligation by exploiting a Ser residue in the native sequences as the +1 position of the MCM2 intein. The split halo-obligatory intein was successfully used to demonstrate the utility in NMR investigation of intact proteins by producing segmentally isotope-labeled intact TonB protein from Helicobacter pylori.

Mycobacteriophages as Incubators for Intein Dissemination and Evolution

Inteins are self-splicing protein elements that are mobile at the DNA level and are sporadically distributed across microbial genomes. Inteins appear to be horizontally transferred, and it has been speculated that phages may play a role in intein distribution. Our attention turns to mycobacteriophages, which infect mycobacteria, where both phage and host harbor inteins. Using bioinformatics, mycobacteriophage genomes were mined for inteins. This study reveals that these mobile elements are present across multiple mycobacteriophage clusters and are pervasive in certain genes, like the large terminase subunit TerL and a RecB-like nuclease, with the majority of intein-containing genes being phage specific. Strikingly, despite this phage specificity, inteins localize to functional motifs shared with bacteria, such that intein-containing genes have similar roles, like hydrolase activity and nucleic acid binding, indicating a global commonality among intein-hosting proteins. Additionally, there are multiple insertion points within active centers, implying independent invasion events, with regulatory implications. Several phage inteins were shown to be splicing competent and to encode functional homing endonucleases, important for mobility. Further, bioinformatic analysis supports the potential for phages as facilitators of intein movement among mycobacteria and related genera. Analysis of catalytic intein residues finds the highly conserved penultimate histidine inconsistently maintained among mycobacteriophages. Biochemical characterization of a noncanonical phage intein shows that this residue influences precursor accumulation, suggesting that splicing has been tuned in phages to modulate generation of important proteins. Together, this work expands our understanding of phage-based intein dissemination and evolution and implies that phages provide a context for evolution of splicing-based regulation.

Inteins are mobile protein splicing elements found in critical genes across all domains of life. Mycobacterial inteins are of particular interest because of their occurrence in pathogenic species, such as Mycobacterium tuberculosis and Mycobacterium leprae, which harbor inteins in important proteins. We have discovered a similarity in activities of intein-containing proteins among mycobacteriophages and their intein-rich actinobacterial hosts, with implications for both posttranslational regulation by inteins and phages participating in horizontal intein transfer. Our demonstration of multiple insertion points within active centers of phage proteins implies independent invasion events, indicating the importance of intein maintenance at specific functional sites. The variable conservation of a catalytic splicing residue, leading to profoundly altered splicing rates, points to the regulatory potential of inteins and to mycobacteriophages playing a role in intein evolution. Collectively, these results suggest inteins as posttranslational regulators and mycobacteriophages as both vehicles for intein distribution and incubators for intein evolution.

Impact of a homing intein on recombination frequency and organismal fitness

Inteins are parasitic genetic elements that excise themselves at the protein level by self-splicing, allowing the formation of functional, nondisrupted proteins. Many inteins contain a homing endonuclease (HEN) domain and rely on its activity for horizontal propagation. However, successful invasion of an entire population will make this activity redundant, and the HEN domain is expected to degenerate quickly under these conditions. Several theories have been proposed for the continued existence of the both active HEN and noninvaded alleles within a population. However, to date, these models were not directly tested experimentally. Using the natural cell fusion ability of the halophilic archaeon Haloferax volcanii we were able to examine this question in vivo, by mating polB intein-positive [insertion site c in the gene encoding DNA polymerase B (polB-c)] and intein-negative cells and examining the dispersal efficiency of this intein in a natural, polyploid population. Through competition between otherwise isogenic intein-positive and intein-negative strains we determined a surprisingly high fitness cost of over 7% for the polB-c intein. Our laboratory culture experiments and samples taken from Israel's Mediterranean coastline show that the polB-c inteins do not efficiently take over an inteinless population through mating, even under ideal conditions. The presence of the HEN/intein promoted recombination when intein-positive and intein-negative cells were mated. Increased recombination due to HEN activity contributes not only to intein dissemination but also to variation at the population level because recombination tracts during repair extend substantially from the homing site.

Salt-Dependent Conditional Protein Splicing of an Intein from Halobacterium salinarum

An intein from Halobacterium salinarum can be isolated as an unspliced precursor protein with exogenous exteins after Escherichia coli overexpression. The intein promotes protein splicing and uncoupled N-terminal cleavage in vitro, conditional on incubation with NaCl or KCl at concentrations of >1.5 M. The protein splicing reaction also is conditional on reduction of a disulfide bond between two active site cysteines. Conditional protein splicing under these relatively mild conditions may lead to advances in intein-based biotechnology applications and hints at the possibility that this H. salinarum intein could serve as a switch to control extein activity under physiologically relevant conditions.

Mobile genetic elements: in silico, in vitro, in vivo

Mobile genetic elements (MGEs), also called transposable elements (TEs), represent universal components of most genomes and are intimately involved in nearly all aspects of genome organization, function and evolution. However, there is currently a gap between the fast pace of TE discovery in silico, driven by the exponential growth of comparative genomic studies, and a limited number of experimental models amenable to more traditional in vitro and in vivo studies of structural, mechanistic and regulatory properties of diverse MGEs. Experimental and computational scientists came together to bridge this gap at a recent conference, 'Mobile Genetic Elements: in silico, in vitro, in vivo', held at the Marine Biological Laboratory (MBL) in Woods Hole, MA, USA.

Intein Clustering Suggests Functional Importance in Different Domains of Life

Inteins, also called protein introns, are self-splicing mobile elements found in all domains of life. A bioinformatic survey of genomic data highlights a biased distribution of inteins among functional categories of proteins in both bacteria and archaea, with a strong preference for a single network of functions containing replisome proteins. Many nonorthologous, functionally equivalent replicative proteins in bacteria and archaea carry inteins, suggesting a selective retention of inteins in proteins of particular functions across domains of life. Inteins cluster not only in proteins with related roles but also in specific functional units of those proteins, like ATPase domains. This peculiar bias does not fully fit the models describing inteins exclusively as parasitic elements. In such models, evolutionary dynamics of inteins is viewed primarily through their mobility with the intein homing endonuclease (HEN) as the major factor of intein acquisition and loss. Although the HEN is essential for intein invasion and spread in populations, HEN dynamics does not explain the observed biased distribution of inteins among proteins in specific functional categories. We propose that the protein splicing domain of the intein can act as an environmental sensor that adapts to a particular niche and could increase the chance of the intein becoming fixed in a population. We argue that selective retention of some inteins might be beneficial under certain environmental stresses, to act as panic buttons that reversibly inhibit specific networks, consistent with the observed intein distribution.

SufB intein of Mycobacterium tuberculosis as a sensor for oxidative and nitrosative stresses

Inteins are mobile genetic elements that self-splice at the protein level. Mycobacteria have inteins inserted into several important genes, including those corresponding to the iron-sulfur cluster assembly protein SufB. Curiously, the SufB inteins are found primarily in mycobacterial species that are potential human pathogens. Here we discovered an exceptional sensitivity of Mycobacterium tuberculosis SufB intein splicing to oxidative and nitrosative stresses when expressed in Escherichia coli. This effect results from predisposition of the intein's catalytic cysteine residues to oxidative and nitrosative modifications. Experiments with a fluorescent reporter system revealed that reactive oxygen species and reactive nitrogen species inhibit SufB extein ligation by forcing either precursor accumulation or N-terminal cleavage. We propose that splicing inhibition is an immediate, posttranslational regulatory response that can be either reversible, by inducing precursor accumulation, or irreversible, by inducing N-terminal cleavage, which may potentially channel mycobacteria into dormancy under extreme oxidative and nitrosative stresses.