Multisubunit RNA polymerases

NTPs compete in the active site of RNA polymerases I and II

Eukaryotes express at least three RNA polymerases (Pols) carry out transcription, while bacteria and archaea use only one. Using transient state kinetics, we have extensively examined and compared the kinetics of both single and multi-nucleotide additions catalyzed by the three Pols. In single nucleotide addition experiments we have observed unexpected extension products beyond one incorporation, which can be attributed to misincorporation, the presence of nearly undetectable amounts of contaminating NTPs, or a mixture of the two. Here we report the development and validation of an analysis strategy to account for the presence of unexpected extension products, when they occur. Using this approach, we uncovered evidence showing that non-cognate nucleotide, thermodynamically, competes with cognate nucleotide for the active site within the elongation complex of Pol I, ΔA12 Pol I, and Pol II. This observation is unexpected because base pairing interactions provide favorable energetics for selectivity and competitive binding indicates that the affinities of cognate and non-cognate nucleotides are within an order of magnitude. Thus, we show that application of our approach will allow for the extraction of additional information that reports on the energetics of nucleotide entry and selectivity.

Higher-order epistasis within Pol II trigger loop haplotypes

RNA polymerase II (Pol II) has a highly conserved domain, the trigger loop (TL), that controls transcription fidelity and speed. We previously probed pairwise genetic interactions between residues within and surrounding the TL for the purpose of understand functional interactions between residues and to understand how individual mutants might alter TL function. We identified widespread incompatibility between TLs of different species when placed in the Saccharomyces cerevisiae Pol II context, indicating species-specific interactions between otherwise highly conserved TLs and its surroundings. These interactions represent epistasis between TL residues and the rest of Pol II. We sought to understand why certain TL sequences are incompatible with S. cerevisiae Pol II and to dissect the nature of genetic interactions within multiply substituted TLs as a window on higher order epistasis in this system. We identified both positive and negative higher-order residue interactions within example TL haplotypes. Intricate higher-order epistasis formed by TL residues was sometimes only apparent from analysis of intermediate genotypes, emphasizing complexity of epistatic interactions. Furthermore, we distinguished TL substitutions with distinct classes of epistatic patterns, suggesting specific TL residues that potentially influence TL evolution. Our examples of complex residue interactions suggest possible pathways for epistasis to facilitate Pol II evolution.

Global kinetic mechanism describing single nucleotide incorporation for RNA polymerase I reveals fast UMP incorporation

RNA polymerase I (Pol I) is responsible for synthesizing ribosomal RNA, which is the rate limiting step in ribosome biogenesis. We have reported wide variability in the magnitude of the rate constants defining the rate limiting step in sequential nucleotide additions catalyzed by Pol I. in this study we sought to determine if base identity impacts the rate limiting step of nucleotide addition catalyzed by Pol I. To this end, we report a transient state kinetic interrogation of AMP, CMP, GMP, and UMP incorporations catalyzed by Pol I. We found that Pol I uses one kinetic mechanism to incorporate all nucleotides. However, we found that UMP incorporation is faster than AMP, CMP, and GMP additions. Further, we found that endonucleolytic removal of a dimer from the 3' end was fastest when the 3' terminal base is a UMP. It has been previously shown that both downstream and upstream template sequence identity impacts the kinetics of nucleotide addition. The results reported here show that the incoming base identity also impacts the magnitude of the observed rate limiting step.

RNA Polymerase II Activity Control of Gene Expression and Involvement in Disease

Gene expression is dependent on RNA Polymerase II (Pol II) activity in eukaryotes. In addition to determining the rate of RNA synthesis for all protein coding genes, Pol II serves as a platform for the recruitment of factors and regulation of co-transcriptional events, from RNA processing to chromatin modification and remodeling. The transcriptome can be shaped by changes in Pol II kinetics affecting RNA synthesis itself or because of alterations to co-transcriptional events that are responsive to or coupled with transcription. Genetic, biochemical, and structural approaches to Pol II in model organisms have revealed critical insights into how Pol II works and the types of factors that regulate it. The complexity of Pol II regulation generally increases with organismal complexity. In this review, we describe fundamental aspects of how Pol II activity can shape gene expression, discuss recent advances in how Pol II elongation is regulated on genes, and how altered Pol II function is linked to human disease and aging.

Reversible Kinetics in Multi-nucleotide Addition Catalyzed by S. cerevisiae RNA polymerase II Reveal Slow Pyrophosphate Release

Eukaryotes express at least three nuclear DNA dependent RNA polymerases (Pols). Pols I, II, and III synthesize ribosomal (r) RNA, messenger (m) RNA, and transfer (t) RNA, respectively. Pol I and Pol III have intrinsic nuclease activity conferred by the A12.2 and C11 subunits, respectively. In contrast, Pol II requires the transcription factor (TF) IIS to confer robust nuclease activity. We recently reported that in the absence of the A12.2 subunit Pol I reverses bond formation by pyrophosphorolysis in the absence of added PPi, indicating slow PPi release. Thus, we hypothesized that Pol II, naturally lacking TFIIS, would reverse bond formation through pyrophosphorolysis. Here we report the results of transient-state kinetic experiments to examine the addition of nine nucleotides to a growing RNA chain catalyzed by Pol II. Our results indicate that Pol II reverses bond formation by pyrophosphorolysis in the absence of added PPi. We propose that, in the absence of endonuclease activity, this bond reversal may represent kinetic proofreading. Thus, given the hypothesis that Pol I evolved from Pol II through the incorporation of general transcription factors, pyrophosphorolysis may represent a more ancient form of proofreading that has been evolutionarily replaced with nuclease activity.

Quantifying the impact of initial RNA primer length on nucleotide addition by RNA polymerase I

Transient state kinetic studies of eukaryotic DNA-dependent RNA polymerases (Pols) in vitro provide quantitative characterization of enzyme activity at the level of individual nucleotide addition events. Previous work revealed heterogeneity in the rate constants governing nucleotide addition by yeast RNA polymerase I (Pol I) for each position on a template DNA. In contrast, the rate constants that described nucleotide addition by yeast RNA polymerase II (Pol II) were more homogeneous. This observation led to the question, what drives the variability of rate constants governing RNA synthesis by Pol I? Are the kinetics of nucleotide addition dictated by the position of the nascent RNA within the polymerase or by the identity of the next encoded nucleotide? In this study, we examine the impact of nucleotide position (i.e. nascent RNA primer length) on the rate constants governing nine sequential nucleotide addition events catalyzed by Pol I. The results reveal a conserved trend in the observed rate constants at each position for all primer lengths used, and highlight that the 9-nucleotide, or 9-mer, RNA primer provides the fastest observed rate constants. These findings suggest that the observed heterogeneity of rate constants for RNA synthesis by Pol I in vitro is driven primarily by the template sequence.

KSHV-encoded LANA bypasses transcriptional block through the stabilization of RNA Pol II in hypoxia

IMPORTANCE

Hypoxia can induce the reactivation of Kaposi sarcoma-associated virus (KSHV), which necessitates the synthesis of critical structural proteins. Despite the unfavorable energetic conditions of hypoxia, KSHV utilizes mechanisms to prevent the degradation of essential cellular machinery required for successful reactivation. Our study provides new insights on strategies employed by KSHV-infected cells to maintain steady-state transcription by overcoming hypoxia-mediated metabolic stress to enable successful reactivation. Our discovery that the interaction of latency-associated nuclear antigen with HIF1α and NEDD4 inhibits its polyubiquitination activity, which blocks the degradation of RNA Pol II during hypoxia, is a significant contribution to our understanding of KSHV biology. This newfound knowledge provides new leads in the development of novel therapies for KSHV-associated diseases.

Small molecule inhibition of RNA binding proteins in haematologic cancer

In recent years, advances in biomedicine have revealed an important role for post-transcriptional mechanisms of gene expression regulation in pathologic conditions. In cancer in general and leukaemia specifically, RNA binding proteins have emerged as important regulator of RNA homoeostasis that are often dysregulated in the disease state. Having established the importance of these pathogenetic mechanisms, there have been a number of efforts to target RNA binding proteins using oligonucleotide-based strategies, as well as with small organic molecules. The field is at an exciting inflection point with the convergence of biomedical knowledge, small molecule screening strategies and improved chemical methods for synthesis and construction of sophisticated small molecules. Here, we review the mechanisms of post-transcriptional gene regulation, specifically in leukaemia, current small-molecule based efforts to target RNA binding proteins, and future prospects.

Molecular basis for proofreading by the unique exonuclease domain of Family-D DNA polymerases

Replicative DNA polymerases duplicate entire genomes at high fidelity. This feature is shared among the three domains of life and is facilitated by their dual polymerase and exonuclease activities. Family D replicative DNA polymerases (PolD), found exclusively in Archaea, contain an unusual RNA polymerase-like catalytic core, and a unique Mre11-like proofreading active site. Here, we present cryo-EM structures of PolD trapped in a proofreading mode, revealing an unanticipated correction mechanism that extends the repertoire of protein domains known to be involved in DNA proofreading. Based on our experimental structures, mutants of PolD were designed and their contribution to mismatch bypass and exonuclease kinetics was determined. This study sheds light on the convergent evolution of structurally distinct families of DNA polymerases, and the domain acquisition and exchange mechanism that occurred during the evolution of the replisome in the three domains of life.

Predictive Potential of RNA Polymerase B (II) Subunit 1 (RPB1) Cytoplasmic Aggregation for Neoadjuvant Chemotherapy Failure

We aimed to investigate the contribution of co-translational protein aggregation to the chemotherapy resistance of tumor cells. Increased co-translational protein aggregation reflects altered translation regulation that may have the potential to buffer transcription under genotoxic stress. As an indicator for such an event, we followed the cytoplasmic aggregation of RPB1, the aggregation-prone largest subunit of RNA polymerase II, in biopsy samples taken from patients with invasive carcinoma of no special type. RPB1 frequently aggregates co-translationally in the absence of proper HSP90 chaperone function or in ribosome mutant cells as revealed formerly in yeast. We found that cytoplasmic foci of RPB1 occur in larger sizes in tumors that showed no regression after therapy. Based on these results, we propose that monitoring the cytoplasmic aggregation of RPB1 may be suitable for determining-from biopsy samples taken before treatment-the effectiveness of neoadjuvant chemotherapy.

RNA Polymerase II evolution and adaptations: Insights from Plasmodium and other parasitic protists

The C-terminal domain (CTD) of RNA polymerase II plays a crucial role in regulating transcription dynamics in eukaryotes. The phosphorylation of serine residues within the CTD controls transcription initiation, elongation, and termination. While the CTD is highly conserved across eukaryotes, lower eukaryotes like protists, including Plasmodium, exhibit some differences. In this study, we performed a comparative analysis of CTD in eukaryotic systems to understand why the parasites evolved in this particular manner. The Plasmodium falciparum RPB1 is exceptionally large and feature a gap between the first and second heptad repeats, resulting in fifteen canonical heptad repeats excluding the initial repeat. Analysis of this intervening sequence revealed sub motifs of heptads where two serine residues occupy the first and fourth positions (S1X2X3S4). These motifs lie in the intrinsically disordered region of RPB1, a characteristic feature of the CTD. Interestingly, the S1X2X3S4 sub-motif was also observed in early-divergingeukaryotes like Leishmania major, which lack canonical heptad repeats. Furthermore, eukaryotes across the phylogenetic tree revealed a sigmoid pattern of increasing serine frequency in the CTD, indicating that serine enrichment is a significant step in the evolution of heptad-rich RPB1. Based on these observations and analysis, we proposed an evolutionary model for RNA Polymerase II CTD, encompassing organisms previously deemed exceptions, notably Plasmodium species. Thus, our study provides novel insights into the evolution of the CTD and will prompt further investigations into the differences exhibited by Plasmodium RNA Pol II and determine if they confer a survival advantage to the parasite.

The A12.2 Subunit Plays an Integral Role in Pyrophosphate Release of RNA Polymerase I

RNA polymerase I (Pol I) synthesizes ribosomal RNA (rRNA), which is the first and rate-limiting step in ribosome biosynthesis. A12.2 (A12) is a critical subunit of Pol I that is responsible for activating Pol I's exonuclease activity. We previously reported a kinetic mechanism for single-nucleotide incorporation catalyzed by Pol I lacking the A12 subunit (ΔA12 Pol I) purified from S. cerevisae and revealed that ΔA12 Pol I exhibited much slower incorporation compared to Pol I. However, it is unknown if A12 influences each nucleotide incorporation in the context of transcription elongation. Here, we show that A12 contributes to every repeating cycle of nucleotide addition and that deletion of A12 results in an entirely different kinetic mechanism compared to WT Pol I. We found that instead of one irreversible step between each nucleotide addition cycle, as reported for wild type (WT) Pol I, the ΔA12 variant requires one reversible step to describe each nucleotide addition. Reversibility fundamentally requires slow PPi release. Consistently, we show that Pol I is more pyrophosphate (PPi) concentration dependent than ΔA12 Pol I. This observation supports the model that PPi is retained in the active site of ΔA12 Pol I longer than WT Pol I. These results suggest that A12 promotes PPi release, revealing a larger role for the A12.2 subunit in the nucleotide addition cycle beyond merely activating exonuclease activity.

Widespread epistasis shapes RNA Polymerase II active site function and evolution

Multi-subunit RNA Polymerases (msRNAPs) are responsible for transcription in all kingdoms of life. At the heart of these msRNAPs is an ultra-conserved active site domain, the trigger loop (TL), coordinating transcription speed and fidelity by critical conformational changes impacting multiple steps in substrate selection, catalysis, and translocation. Previous studies have observed several different types of genetic interactions between eukaryotic RNA polymerase II (Pol II) TL residues, suggesting that the TL's function is shaped by functional interactions of residues within and around the TL. The extent of these interaction networks and how they control msRNAP function and evolution remain to be determined. Here we have dissected the Pol II TL interaction landscape by deep mutational scanning in Saccharomyces cerevisiae Pol II. Through analysis of over 15000 alleles, representing all single mutants, a rationally designed subset of double mutants, and evolutionarily observed TL haplotypes, we identify interaction networks controlling TL function. Substituting residues creates allele-specific networks and propagates epistatic effects across the Pol II active site. Furthermore, the interaction landscape further distinguishes alleles with similar growth phenotypes, suggesting increased resolution over the previously reported single mutant phenotypic landscape. Finally, co-evolutionary analyses reveal groups of co-evolving residues across Pol II converge onto the active site, where evolutionary constraints interface with pervasive epistasis. Our studies provide a powerful system to understand the plasticity of RNA polymerase mechanism and evolution, and provide the first example of pervasive epistatic landscape in a highly conserved and constrained domain within an essential enzyme.

Discovery, implications and initial use of semi-synthetic organisms with an expanded genetic alphabet/code

Much recent interest has focused on developing proteins for human use, such as in medicine. However, natural proteins are made up of only a limited number of canonical amino acids with limited functionalities, and this makes the discovery of variants with some functions difficult. The ability to recombinantly express proteins containing non-canonical amino acids (ncAAs) with properties selected to impart the protein with desired properties is expected to dramatically improve the discovery of proteins with different functions. Perhaps the most straightforward approach to such an expansion of the genetic code is through expansion of the genetic alphabet, so that new codon/anticodon pairs can be created to assign to ncAAs. In this review, I briefly summarize more than 20 years of effort leading ultimately to the discovery of synthetic nucleotides that pair to form an unnatural base pair, which when incorporated into DNA, is stably maintained, transcribed and used to translate proteins in Escherichia coli. In addition to discussing wide ranging conceptual implications, I also describe ongoing efforts at the pharmaceutical company Sanofi to employ the resulting 'semi-synthetic organisms' or SSOs, for the production of next-generation protein therapeutics. This article is part of the theme issue 'Reactivity and mechanism in chemical and synthetic biology'.

Investigation of Multi-Subunit Mycobacterium tuberculosis DNA-Directed RNA Polymerase and Its Rifampicin Resistant Mutants

Emerging Mycobacterium tuberculosis (Mtb) resistant strains have continued to limit the efficacies of existing antitubercular therapies. More specifically, mutations in the RNA replicative machinery of Mtb, RNA polymerase (RNAP), have been widely linked to rifampicin (RIF) resistance, which has led to therapeutic failures in many clinical cases. Moreover, elusive details on the underlying mechanisms of RIF-resistance caused by Mtb-RNAP mutations have hampered the development of new and efficient drugs that are able to overcome this challenge. Therefore, in this study we attempt to resolve the molecular and structural events associated with RIF-resistance in nine clinically reported missense Mtb RNAP mutations. Our study, for the first time, investigated the multi-subunit Mtb RNAP complex and findings revealed that the mutations commonly disrupted structural-dynamical attributes that may be essential for the protein's catalytic functions, particularly at the βfork loop 2, β'zinc-binding domain, the β' trigger loop and β'jaw, which in line with previous experimental reports, are essential for RNAP processivity. Complementarily, the mutations considerably perturbed the RIF-BP, which led to alterations in the active orientation of RIF needed to obstruct RNA extension. Consequentially, essential interactions with RIF were lost due to the mutation-induced repositioning with corresponding reductions in the binding affinity of the drug observed in majority of the mutants. We believe these findings will significantly aid future efforts in the discovery of new treatment options with the potential to overcome antitubercular resistance.

Three-Dimensional Envelope and Subunit Interactions of the Plastid-Encoded RNA Polymerase from Sinapis alba

RNA polymerases (RNAPs) are found in all living organisms. In the chloroplasts, the plastid-encoded RNA polymerase (PEP) is a prokaryotic-type multimeric RNAP involved in the selective transcription of the plastid genome. One of its active states requires the assembly of nuclear-encoded PEP-Associated Proteins (PAPs) on the catalytic core, producing a complex of more than 900 kDa, regarded as essential for chloroplast biogenesis. In this study, sequence alignments of the catalytic core subunits across various chloroplasts of the green lineage and prokaryotes combined with structural data show that variations are observed at the surface of the core, whereas internal amino acids associated with the catalytic activity are conserved. A purification procedure compatible with a structural analysis was used to enrich the native PEP from Sinapis alba chloroplasts. A mass spectrometry (MS)-based proteomic analysis revealed the core components, the PAPs and additional proteins, such as FLN2 and pTAC18. MS coupled with crosslinking (XL-MS) provided the initial structural information in the form of protein clusters, highlighting the relative position of some subunits with the surfaces of their interactions. Using negative stain electron microscopy, the PEP three-dimensional envelope was calculated. Particles classification shows that the protrusions are very well-conserved, offering a framework for the future positioning of all the PAPs. Overall, the results show that PEP-associated proteins are firmly and specifically associated with the catalytic core, giving to the plastid transcriptional complex a singular structure compared to other RNAPs.

Structural advances in transcription elongation

Transcription is the first step of gene expression and involves RNA polymerases. After transcription initiation, RNA polymerase enters elongation followed by transcription termination at the end of the gene. Only recently, structures of transcription elongation complexes bound to key transcription elongation factors have been determined in bacterial and eukaryotic systems. These structures have revealed numerous insights including the basis for transcriptional pausing, RNA polymerase interaction with large complexes such as the ribosome and the spliceosome, and the transition into productive elongation. Here, we review these structures and describe areas for future research.

Widespread association of ERα with RMRP and tRNA genes in MCF-7 cells and breast cancers

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Estrogen receptor (ER) interacts with hundreds of tRNA genes (tDNAs) in MCF-7 cells.

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Hundreds of tDNAs are also targeted in primary breast tumours and metastases.

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Canonical estrogen response element is not found near top tDNA targets of ER.

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ER also targets non-coding breast cancer driver gene RMRP.

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ER also targets RN7SL1 gene that promotes breast cancer progression.

tRNA gene transcription by RNA polymerase III (Pol III) is a tightly regulated process, but dysregulated Pol III transcription is widely observed in cancers. Approximately 75% of all breast cancers are positive for expression of Estrogen Receptor alpha (ERα), which acts as a key driver of disease. MCF-7 cells rapidly upregulate tRNA gene transcription in response to estrogen and ChIP-PCR demonstrated ERα enrichment at tRNA^Leu and 5S rRNA genes in this breast cancer cell line. While these data implicate the ERα as a Pol III transcriptional regulator, how widespread this regulation is across the 631 tRNA genes has yet to be revealed. Through analyses of ERα ChIP-seq datasets, we show that ERα interacts with hundreds of tRNA genes, not only in MCF-7 cells, but also in primary human breast tumours and distant metastases. The extent of ERα association with tRNA genes varies between breast cancer cell lines and does not correlate with levels of ERα binding to its canonical target gene GREB1. Amongst other Pol III-transcribed genes, ERα is consistently enriched at the long non-coding RNA gene RMRP, a positive regulator of cell cycle progression that is subject to focal amplification in tumours. Another Pol III template targeted by ERα is the RN7SL1 gene, which is strongly implicated in breast cancer pathology by inducing inflammatory responses in tumours. Our data indicate that Pol III-transcribed non-coding genes should be added to the list of ERα targets in breast cancer.

Rapid Exchange of Stably Bound Protein and DNA Cargo on a DNA Origami Receptor

Biomolecular complexes can form stable assemblies yet can also rapidly exchange their subunits to adapt to environmental changes. Simultaneously allowing for both stability and rapid exchange expands the functional capacity of biomolecular machines and enables continuous function while navigating a complex molecular world. Inspired by biology, we design and synthesize a DNA origami receptor that exploits multivalent interactions to form stable complexes that are also capable of rapid subunit exchange. The system utilizes a mechanism first outlined in the context of the DNA replisome, known as multisite competitive exchange, and achieves a large separation of time scales between spontaneous subunit dissociation, which requires days, and rapid subunit exchange, which occurs in minutes. In addition, we use the DNA origami receptor to demonstrate stable interactions with rapid exchange of both DNA and protein subunits, thus highlighting the applicability of our approach to arbitrary molecular cargo, an important distinction with canonical toehold exchange between single-stranded DNA. We expect this study to benefit future studies that use DNA origami structures to exploit multivalent interactions for the design and synthesis of a wide range of possible kinetic behaviors.

Switching promotor recognition of phage RNA polymerase in silico along lab-directed evolution path

In this work, we computationally investigated how a viral RNA polymerase (RNAP) from bacteriophage T7 evolves into RNAP variants under lab-directed evolution to switch recognition from T7 promoter to T3 promoter in transcription initiation. We first constructed a closed initiation complex for the wild-type T7 RNAP and then for six mutant RNAPs discovered from phage-assisted continuous evolution experiments. All-atom molecular dynamics simulations up to 1 μs each were conducted on these RNAPs in a complex with the T7 and T3 promoters. Our simulations show notably that protein-DNA electrostatic interactions or stabilities at the RNAP-DNA promoter interface well dictate the promoter recognition preference of the RNAP and variants. Key residues and structural elements that contribute significantly to switching the promoter recognition were identified. Followed by a first point mutation N748D on the specificity loop to slightly disengage the RNAP from the promoter to hinder the original recognition, we found an auxiliary helix (206-225) that takes over switching the promoter recognition upon further mutations (E222K and E207K) by forming additional charge interactions with the promoter DNA and reorientating differently on the T7 and T3 promoters. Further mutations on the AT-rich loop and the specificity loop can fully switch the RNAP-promoter recognition to the T3 promoter. Overall, our studies reveal energetics and structural dynamics details along an exemplary directed evolutionary path of the phage RNAP variants for a rewired promoter recognition function. The findings demonstrate underlying physical mechanisms and are expected to assist knowledge and data learning or rational redesign of the protein enzyme structure function.

Carbon catabolite repression involves physical interaction of the transcription factor CRE1/CreA and the Tup1-Cyc8 complex in Penicillium oxalicum and Trichoderma reesei

BACKGROUND

Cellulolytic enzyme production in filamentous fungi requires a release from carbon catabolite repression (CCR). The protein CRE1/CreA (CRE = catabolite responsive element) is a key transcription factor (TF) that is involved in CCR and represses cellulolytic gene expression. CRE1/CreA represents the functional equivalent of Mig1p, an important Saccharomyces cerevisiae TF in CCR that exerts its repressive effect by recruiting a corepressor complex Tup1p-Cyc8p. Although it is known from S. cerevisiae that CRE1/CreA might repress gene expression via interacting with the corepressor complex Tup1-Cyc8, this mechanism is unconfirmed in other filamentous fungi, since the physical interaction has not yet been verified in these organisms. The precise mechanism on how CRE1/CreA achieves transcriptional repression after DNA binding remains unknown.

RESULTS

The results from tandem affinity purification and bimolecular fluorescence complementation revealed a direct physical interaction between the TF CRE1/CreA and the complex Tup1-Cyc8 in the nucleus of cellulolytic fungus Trichoderma reesei and Penicillium oxalicum. Both fungi have the ability to secrete a complex arsenal of enzymes to synergistically degrade lignocellulosic materials. In P. oxalicum, the protein PoCyc8, a subunit of complex Tup1-Cyc8, interacts directly with TF PoCreA and histone H3 lysine 36 (H3K36) methyltransferase PoSet2 in the nucleus. The di-methylation level of H3K36 in the promoter of prominent cellulolytic genes (cellobiohydrolase-encoding gene Pocbh1/cel7A and endoglucanase-encoding gene Poegl1/cel7B) is positively correlated with the expression levels of TF PoCreA. Since the methylation of H3K36 was also demonstrated to be a repression marker of cellulolytic gene expression, it appears feasible that the cellulolytic genes are repressed via PoCreA-Tup1-Cyc8-Set2-mediated transcriptional repression.

CONCLUSION

This study verifies the long-standing conjecture that the TF CRE1/CreA represses gene expression by interacting with the corepressor complex Tup1-Cyc8 in filamentous fungi. A reasonable explanation is proposed that PoCreA represses gene expression by recruiting complex PoTup1-Cyc8. Histone methyltransferase Set2, which methylates H3K36, is also involved in the regulatory network by interacting with PoCyc8. The findings contribute to the understanding of CCR mechanism in filamentous fungi and could aid in biotechnologically relevant enzyme production.

Conserved Trigger Loop Histidine of RNA Polymerase II Functions as a Positional Catalyst Primarily through Steric Effects

In all domains of life, multisubunit RNA polymerases (RNAPs) catalyze both the extension of mRNA transcripts by nucleotide addition and the hydrolysis of RNA, which enables proofreading by removal of misincorporated nucleotides. A highly conserved catalytic module within RNAPs called the trigger loop (TL) functions as the key controller of these activities. The TL is proposed to act as a positional catalyst of phosphoryl transfer and transcript cleavage via electrostatic and steric contacts with substrates in its folded helical form. The function of a near-universally conserved TL histidine that contacts NTP phosphates is of particular interest. Despite its exceptional conservation, substitutions of the TL His with Gln support efficient catalysis in bacterial and yeast RNAPs. Unlike bacterial TLs, which contain a nearby Arg, the TL His is the only acid-base catalyst candidate in the eukaryotic RNAPII TL. Nonetheless, replacement of the TL His with Leu is reported to support cell growth in yeast, suggesting that even hydrogen bonding and polarity at this position may be dispensable for efficient catalysis by RNAPII. To test how a TL His-to-Leu substitution affects the enzymatic functions of RNAPII, we compared its rates of nucleotide addition, pyrophosphorolysis, and RNA hydrolysis to those of the wild-type RNAPII enzyme. The His-to-Leu substitution slightly reduced rates of phosphoryl transfer with little if any effect on intrinsic transcript cleavage. These findings indicate that the highly conserved TL His is neither an obligate acid-base catalyst nor a polar contact for NTP phosphates but instead functions as a positional catalyst mainly through steric effects.

Efforts toward Further Integration of an Unnatural Base Pair into the Biology of a Semisynthetic Organism

We have developed semisynthetic organisms (SSOs) that by virtue of a family of synthetic, unnatural base pairs (UBPs), store and retrieve increased information. To date, transcription in the SSOs has relied on heterologous expression of the RNA polymerase from T7 bacteriophage; here, we explore placing transcription under the control of the endogenous host multisubunit RNA polymerase. The results demonstrate that the E. coli RNA polymerase is able to transcribe DNA containing a UBP and that with the most optimal UBP identified to date it should be possible to select for increased uptake of unnatural triphosphates. These advances should facilitate the creation of next generation SSOs.

Biogenesis of RNA Polymerases in Yeast

Eukaryotic RNA polymerases (RNA pols) transcriptional processes have been extensively investigated, and the structural analysis of eukaryotic RNA pols has been explored. However, the global assembly and biogenesis of these heteromultimeric complexes have been narrowly studied. Despite nuclear transcription being carried out by three RNA polymerases in eukaryotes (five in plants) with specificity in the synthesis of different RNA types, the biogenesis process has been proposed to be similar, at least for RNA pol II, to that of bacteria, which contains only one RNA pol. The formation of three different interacting subassembly complexes to conform the complete enzyme in the cytoplasm, prior to its nuclear import, has been assumed. In Saccharomyces cerevisiae, recent studies have examined in depth the biogenesis of RNA polymerases by characterizing some elements involved in the assembly of these multisubunit complexes, some of which are conserved in humans. This study reviews the latest studies governing the mechanisms and proteins described as being involved in the biogenesis of RNA polymerases in yeast.

Understanding transcription across scales: From base pairs to chromosomes

The influence of genome organization on transcription is central to our understanding of cell type specification. Higher-order genome organization is established through short- and long-range DNA interactions. Coordination of these interactions, from single atoms to entire chromosomes, plays a fundamental role in transcriptional control of gene expression. Loss of this coupling can result in disease. Analysis of transcriptional regulation typically involves disparate experimental approaches, from structural studies that define angstrom-level interactions to cell-biological and genomic approaches that assess mesoscale relationships. Thus, to fully understand the mechanisms that regulate gene expression, it is critical to integrate the findings gained across these distinct size scales. In this review, I illustrate fundamental ways in which cells regulate transcription in the context of genome organization.

Expression attenuation as a mechanism of robustness against gene duplication

Gene duplication is ubiquitous and a major driver of phenotypic diversity across the tree of life, but its immediate consequences are not fully understood. Deleterious effects would decrease the probability of retention of duplicates and prevent their contribution to long-term evolution. One possible detrimental effect of duplication is the perturbation of the stoichiometry of protein complexes. Here, we measured the fitness effects of the duplication of 899 essential genes in the budding yeast using high-resolution competition assays. At least 10% of genes caused a fitness disadvantage when duplicated. Intriguingly, the duplication of most protein complex subunits had small to nondetectable effects on fitness, with few exceptions. We selected four complexes with subunits that had an impact on fitness when duplicated and measured the impact of individual gene duplications on their protein-protein interactions. We found that very few duplications affect both fitness and interactions. Furthermore, large complexes such as the 26S proteasome are protected from gene duplication by attenuation of protein abundance. Regulatory mechanisms that maintain the stoichiometric balance of protein complexes may protect from the immediate effects of gene duplication. Our results show that a better understanding of protein regulation and assembly in complexes is required for the refinement of current models of gene duplication.

Defining the divergent enzymatic properties of RNA polymerases I and II

Eukaryotes express at least three nuclear DNA-dependent RNA polymerases (Pols) responsible for synthesizing all RNA required by the cell. Despite sharing structural homology, they have functionally diverged to suit their distinct cellular roles. Although the Pols have been studied extensively, direct comparison of their enzymatic properties is difficult because studies are often conducted under disparate experimental conditions and techniques. Here, we directly compare and reveal functional differences between Saccharomyces cerevisiae Pols I and II using a series of quantitative in vitro transcription assays. We find that Pol I single-nucleotide and multinucleotide addition rate constants are faster than those of Pol II. Pol I elongation complexes are less stable than Pol II elongation complexes, and Pol I is more error prone than Pol II. Collectively, these data show that the enzymatic properties of the Pols have diverged over the course of evolution, optimizing these enzymes for their unique cellular responsibilities.

Novel ribonucleotide discrimination in the RNA polymerase-like two-barrel catalytic core of Family D DNA polymerases

Family D DNA polymerase (PolD) is the essential replicative DNA polymerase for duplication of most archaeal genomes. PolD contains a unique two-barrel catalytic core absent from all other DNA polymerase families but found in RNA polymerases (RNAPs). While PolD has an ancestral RNA polymerase catalytic core, its active site has evolved the ability to discriminate against ribonucleotides. Until now, the mechanism evolved by PolD to prevent ribonucleotide incorporation was unknown. In all other DNA polymerase families, an active site steric gate residue prevents ribonucleotide incorporation. In this work, we identify two consensus active site acidic (a) and basic (b) motifs shared across the entire two-barrel nucleotide polymerase superfamily, and a nucleotide selectivity (s) motif specific to PolD versus RNAPs. A novel steric gate histidine residue (H931 in Thermococcus sp. 9°N PolD) in the PolD s-motif both prevents ribonucleotide incorporation and promotes efficient dNTP incorporation. Further, a PolD H931A steric gate mutant abolishes ribonucleotide discrimination and readily incorporates a variety of 2' modified nucleotides. Taken together, we construct the first putative nucleotide bound PolD active site model and provide structural and functional evidence for the emergence of DNA replication through the evolution of an ancestral RNAP two-barrel catalytic core.

Structural Biology of RNA Polymerase II Transcription: 20 Years On

Gene transcription by RNA polymerase II (Pol II) is the first step in the expression of the eukaryotic genome and a focal point for cellular regulation during development, differentiation, and responses to the environment. Two decades after the determination of the structure of Pol II, the mechanisms of transcription have been elucidated with studies of Pol II complexes with nucleic acids and associated proteins. Here we provide an overview of the nearly 200 available Pol II complex structures and summarize how these structures have elucidated promoter-dependent transcription initiation, promoter-proximal pausing and release of Pol II into active elongation, and the mechanisms that Pol II uses to navigate obstacles such as nucleosomes and DNA lesions. We predict that future studies will focus on how Pol II transcription is interconnected with chromatin transitions, RNA processing, and DNA repair.

Enzymatic Protein Biopolymers as a Tool to Synthetize Eukaryotic Messenger Ribonucleic Acid (mRNA) with Uses in Vaccination, Immunotherapy and Nanotechnology

Multi-subunit enzymes are protein biopolymers that are involved in many cellular processes. The enzyme that carries out the process of transcription of mRNAs is RNA polymerase II (RNAPII), which is a multi-subunit enzyme in eukaryotes. This protein biopolymer starts the transcription from specific sites and is positioned by transcription factors, which form a preinitiation complex (PIC) on gene promoters. To recognize and position the RNAPII and the transcription factors on the gene promoters are needed specific DNA sequences in the gene promoters, which are named promoter elements. Those gene promoter elements can vary and therefore several kinds of promoters exist, however, it appears that all promoters can use a similar pathway for PIC formation. Those pathways are discussed in this review. The in vitro transcribed mRNA can be used as vaccines to fight infectious diseases, e.g., in immunotherapy against cancer and in nanotechnology to deliver mRNA for a missing protein into the cell. We have outlined a procedure to produce an mRNA vaccine against the SARS-CoV-2 virus, which is the causing agent of the big pandemic, COVID-19, affecting human beings all over the world. The potential advantages of using eukaryotic RNAPII to synthetize large transcripts are outlined and discussed. In addition, we suggest a method to cap the mRNA at the 5′ terminus by using enzymes, which might be more effective than cap analogs. Finally, we suggest the construction of a future multi-talented RNAPII, which would be able to synthetize large mRNA and cap them in the test tube.

RNA polymerase II phosphorylation and gene looping: new roles for the Rpb4/7 heterodimer in regulating gene expression

In eukaryotes, cellular RNAs are produced by three nuclear RNA polymerases (RNAPI, II, and III), which are multisubunit complexes. They share structural and functional features, although they are specialized in the synthesis of specific RNAs. RNAPII transcribes the vast majority of cellular RNAs, including mRNAs and a large number of noncoding RNAs. The structure of RNAPII is highly conserved in all eukaryotes, consisting of 12 subunits (Rpb1-12) organized into five structural modules, among which the Rpb4 and Rpb7 subunits form the stalk. Early studies suggested an accessory role for Rpb4, because is required for specific gene transcription pathways. Far from this initial hypothesis, it is now well established that the Rpb4/7 heterodimer plays much wider roles in gene expression regulation. It participates in nuclear and cytosolic processes ranging from transcription to translation and mRNA degradation in a cyclical process. For this reason, Rpb4/7 is considered a coordinator of gene expression. New functions have been added to the list of stalk functions during transcription, which will be reviewed herein: first, a role in the maintenance of proper RNAPII phosphorylation levels, and second, a role in the establishment of a looped gene architecture in actively transcribed genes.

Visualization of two architectures in class-II CAP-dependent transcription activation

Transcription activation by cyclic AMP (cAMP) receptor protein (CAP) is the classic paradigm of transcription regulation in bacteria. CAP was suggested to activate transcription on class-II promoters via a recruitment and isomerization mechanism. However, whether and how it modifies RNA polymerase (RNAP) to initiate transcription remains unclear. Here, we report cryo–electron microscopy (cryo-EM) structures of an intact Escherichia coli class-II CAP-dependent transcription activation complex (CAP-TAC) with and without de novo RNA transcript. The structures reveal two distinct architectures of TAC and raise the possibility that CAP binding may induce substantial conformational changes in all the subunits of RNAP and transiently widen the main cleft of RNAP to facilitate DNA promoter entering and formation of the initiation open complex. These structural changes vanish during further RNA transcript synthesis. The observations in this study may reveal a possible on-pathway intermediate and suggest a possibility that CAP activates transcription by inducing intermediate state, in addition to the previously proposed stabilization mechanism.

Cryo-EM structures of transcription activation complexes comprising cyclic AMP receptor protein (CAP) bound to a class-II promoter reveal two distinct architectures and suggest a possibility that CAP activates transcription by inducing an intermediate state, an important supplement to the previously proposed stabilization mechanism.

Prokaryotic sigma factors and their transcriptional counterparts in Archaea and Eukarya

RNA polymerases (RNAPs) carry out transcription in the three domains of life, Bacteria, Archaea, and Eukarya. Transcription initiation is highly regulated by a variety of transcription factors, whose number and subunit complexity increase during evolution. This process is regulated in Bacteria by the σ factor, while the three eukaryotic RNAPs require a complex set of transcription factors (TFs) and a TATA-binding protein (TBP). The archaeal transcription system appears to be an ancestral version of the eukaryotic RNAPII, requiring transcription factor B (TFB), TBP, and transcription factor E (TFE). The function of the bacterial sigma (σ) factor has been correlated to the roles played by the eukaryotic RNAP II and the archaeal RNAP. In addition, σ factors, TFB, and TFIIB all contain multiple DNA binding helix-turn-helix (HTH) structural motifs; although TFIIB and TFB display two HTH domains, while the bacterial σ factor spans 4 HTH motifs. The sequence similarities and structure alignments of the bacterial σ factor, eukaryotic TFIIB, and archaeal TFB evidence that these three proteins are homologs.Key Points• Transcription initiation is highly regulated by TFs.• Transcription is finely regulated in all domains of life by different sets of TFs.• Specific TFs in Bacteria, Eukarya and Archaea are homologs.

Mechanism of long noncoding RNAs as transcriptional regulators in cancer

Dysregulation of gene expression, often interpreted by gene transcription as an endpoint response, is tightly associated with human cancer. Long noncoding RNAs (lncRNAs), derived from the noncoding elements in the genome and appeared no less than 200nt in length, have emerged as a novel class of pivotal regulatory component. Recently, great attention has been paid to the cancer-related lncRNAs and growing evidence have shown that lncRNAs act as key transcriptional regulators in cancer cells through diverse mechanisms. Here, we focus on the nucleus-expressed lncRNAs and summarize their molecular mechanisms in transcriptional control during tumorigenesis and cancer metastasis. Six major mechanisms will be discussed in this review: association with transcriptional factor, modulating DNA methylation or histone modification enzyme, influencing on chromatin remodelling complex, facilitating chromosomal looping, interaction with RNA polymerase and direct association with promoter.

5-Formylcytosine-induced DNA-peptide cross-links reduce transcription efficiency, but do not cause transcription errors in human cells

5-Formylcytosine (5fC) is an endogenous epigenetic DNA mark introduced via enzymatic oxidation of 5-methyl-dC in DNA. We and others recently reported that 5fC can form reversible DNA-protein conjugates with histone proteins, likely contributing to regulation of nucleosomal organization and gene expression. The protein component of DNA-protein cross-links can be proteolytically degraded, resulting in smaller DNA-peptide cross-links. Unlike full-size DNA-protein cross-links that completely block replication and transcription, DNA-peptide cross-links can be bypassed by DNA and RNA polymerases and can potentially be repaired via the nucleotide excision repair (NER) pathway. In the present work, we constructed plasmid molecules containing reductively stabilized, site-specific 5fC-polypeptide lesions and employed a quantitative MS-based assay to assess their effects on transcription in cells. Our results revealed that the presence of DNA-peptide cross-link significantly inhibits transcription in human HEK293T cells but does not induce transcription errors. Furthermore, transcription efficiency was similar in WT and NER-deficient human cell lines, suggesting that the 5fC-polypeptide lesion is a weak substrate for NER. This finding was confirmed by in vitro NER assays in cell-free extracts from human HeLa cells, suggesting that another mechanism is required for 5fC-polypeptide lesion removal. In summary, our findings indicate that 5fC-mediated DNA-peptide cross-links dramatically reduce transcription efficiency, are poor NER substrates, and do not cause transcription errors.

The A12.2 Subunit Is an Intrinsic Destabilizer of the RNA Polymerase I Elongation Complex

Despite sharing a highly conserved core architecture with their prokaryotic counterparts, eukaryotic multisubunit RNA polymerases (Pols) have undergone structural divergence and biological specialization. Interesting examples of structural divergence are the A12.2 and C11 subunits of Pols I and III, respectively. Whereas all known cellular Pols possess cognate protein factors that stimulate cleavage of the nascent RNA, Pols I and III have incorporated their cleavage factors as bona fide subunits. Although it is not yet clear why these polymerases have incorporated their cleavage factors as subunits, a picture is emerging that identifies roles for these subunits beyond providing nucleolytic activity. Specifically, it appears that both A12.2 and C11 are required for efficient termination of transcription by Pols I and III. Given that termination involves destabilization of the elongation complex (EC), we tested whether A12.2 influences stability of the Pol I EC. Using, to our knowledge, a novel assay to measure EC dissociation kinetics, we have determined that A12.2 is an intrinsic destabilizer of the Pol I EC. In addition, the salt concentration dependence of Pol I EC dissociation kinetics suggests that A12.2 alters electrostatic interactions within the EC. Importantly, these data present a mechanistic basis for the requirement of A12.2 in Pol I termination. Combined with recent work demonstrating the direct involvement of A12.2 in Pol I nucleotide incorporation, this study further supports the concept that A12.2 cannot be viewed solely as a cleavage factor.

The Extended "Two-Barrel" Polymerases Superfamily: Structure, Function and Evolution

DNA and RNA polymerases (DNAP and RNAP) play central roles in genome replication, maintenance and repair, as well as in the expression of genes through their transcription. Multisubunit RNAPs carry out transcription and are represented, without exception, in all cellular life forms as well as in nucleo-cytoplasmic DNA viruses. Since their discovery, multisubunit RNAPs have been the focus of intense structural and functional studies revealing that they all share a well-conserved active-site region called the two-barrel catalytic core. The two-barrel core hosts the polymerase active site, which is located at the interface between two double-psi β-barrel domains that contribute distinct amino acid residues to the active site in an asymmetrical fashion. Recently, sequencing and structural studies have added a surprising variety of DNA and RNA to the two-barrel superfamily, including the archaeal replicative DNAP (PolD), which extends the family to DNA-dependent DNAPs involved in replication. While all these polymerases share a minimal core that must have been present in their common ancestor, the two-barrel polymerase superfamily now encompasses a remarkable diversity of enzymes, including DNA-dependent RNAPs, RNA-dependent RNAPs, and DNA-dependent DNAPs, which participate in critical biological processes such as DNA transcription, DNA replication, and gene silencing. The present review will discuss both common features and differences among the extended two-barrel polymerase superfamily, focusing on the newly discovered members. Comparing their structures provides insights into the molecular mechanisms evolved by the contemporary two-barrel polymerases to accomplish their different biological functions.

It's all about the T: transcription termination in archaea

One of the most fundamental biological processes driving all life on earth is transcription. The, at first glance, relatively simple cycle is divided into three stages: initiation at the promoter site, elongation throughout the open reading frame, and finally termination and product release at the terminator. In all three processes, motifs of the template DNA and protein factors of the transcription machinery including the multisubunit polymerase itself as well as a broad range of associated transcription factors work together and mutually influence each other. Despite several decades of research, this interplay holds delicate mechanistic and structural details as well as interconnections yet to be explored. One of the surprising characteristics of archaeal biology is the use of eukaryotic-like information processing systems against a backdrop of a bacterial-like genome. Archaeal genomes usually comprise main chromosomes alongside chromosomal plasmids, and the genetic information is encoded in single transcriptional units as well as in multicistronic operons alike their bacterial counterparts. Moreover, archaeal genomes are densely packed and this necessitates a tight regulation of transcription and especially assured termination events in order to prevent read-through into downstream coding regions and the accumulation of antisense transcripts.

Delayed inhibition mechanism for secondary channel factor regulation of ribosomal RNA transcription

RNA polymerases (RNAPs) contain a conserved ‘secondary channel’ which binds regulatory factors that modulate transcription initiation. In Escherichia coli, the secondary channel factors (SCFs) GreB and DksA both repress ribosomal RNA (rRNA) transcription, but SCF loading and repression mechanisms are unclear. We observed in vitro fluorescently labeled GreB molecules binding to single RNAPs and initiation of individual transcripts from an rRNA promoter. GreB arrived and departed from promoters only in complex with RNAP. GreB did not alter initial RNAP-promoter binding but instead blocked a step after conformational rearrangement of the initial RNAP-promoter complex. Strikingly, GreB-RNAP complexes never initiated at an rRNA promoter; only RNAP molecules arriving at the promoter without bound GreB produced transcript. The data reveal that a model SCF functions by a ‘delayed inhibition’ mechanism and suggest that rRNA promoters are inhibited by GreB/DksA because their short-lived RNAP complexes do not allow sufficient time for SCFs to dissociate.

The Ribosome as a Missing Link in Prebiotic Evolution III: Over-Representation of tRNA- and rRNA-Like Sequences and Plieofunctionality of Ribosome-Related Molecules Argues for the Evolution of Primitive Genomes from Ribosomal RNA Modules

We propose that ribosomal RNA (rRNA) formed the basis of the first cellular genomes, and provide evidence from a review of relevant literature and proteonomic tests. We have proposed previously that the ribosome may represent the vestige of the first self-replicating entity in which rRNAs also functioned as genes that were transcribed into functional messenger RNAs (mRNAs) encoding ribosomal proteins. rRNAs also encoded polymerases to replicate itself and a full complement of the transfer RNAs (tRNAs) required to translate its genes. We explore here a further prediction of our “ribosome-first” theory: the ribosomal genome provided the basis for the first cellular genomes. Modern genomes should therefore contain an unexpectedly large percentage of tRNA- and rRNA-like modules derived from both sense and antisense reading frames, and these should encode non-ribosomal proteins, as well as ribosomal ones with key cell functions. Ribosomal proteins should also have been co-opted by cellular evolution to play extra-ribosomal functions. We review existing literature supporting these predictions. We provide additional, new data demonstrating that rRNA-like sequences occur at significantly higher frequencies than predicted on the basis of mRNA duplications or randomized RNA sequences. These data support our “ribosome-first” theory of cellular evolution.

Dual coding potential of a 2',5'-branched ribonucleotide in DNA

Branchpoints in RNA templates are highly mutagenic, but it is not known yet whether this also applies to branchpoints in DNA templates. Here, we report how nucleic acid polymerases replicate a 2',5'-branched DNA (bDNA) molecule. We constructed long-chained bDNA templates containing a branch guanosine and T7 promoters at both arms by splinted ligation. Quantitative real-time PCR analysis was used to investigate whether a branchpoint blocks DNA synthesis from the two arms in the same manner. We find that the blocking effect of a branchpoint is arm-specific. DNA synthesis from the 2'-arm is more than 20,000-fold decreased, whereas from the 3'-arm only 15-fold. Our sequence analysis of full-length nucleic acid generated by Taq DNA polymerase, Moloney murine leukemia virus reverse transcriptase, and T7 RNA polymerase from the 2'-arm of bDNA shows that the branched guanine has a dual coding potential and can base-pair with cytosine and guanine. We find that branchpoint templating is influenced by the type of the surrounding nucleic acid and is probably modulated by polymerase and RNase H active sites. We show that the branchpoint bypass by the polymerases from the 3'-arm of bDNA is predominantly error-free, indicating that bDNA is not as highly mutagenic as 2',5'-branched RNA.

Highly efficient 5' capping of mitochondrial RNA with NAD+ and NADH by yeast and human mitochondrial RNA polymerase

Bacterial and eukaryotic nuclear RNA polymerases (RNAPs) cap RNA with the oxidized and reduced forms of the metabolic effector nicotinamide adenine dinucleotide, NAD⁺ and NADH, using NAD⁺ and NADH as non-canonical initiating nucleotides for transcription initiation. Here, we show that mitochondrial RNAPs (mtRNAPs) cap RNA with NAD⁺ and NADH, and do so more efficiently than nuclear RNAPs. Direct quantitation of NAD⁺- and NADH-capped RNA demonstrates remarkably high levels of capping in vivo: up to ~60% NAD⁺ and NADH capping of yeast mitochondrial transcripts, and up to ~15% NAD⁺ capping of human mitochondrial transcripts. The capping efficiency is determined by promoter sequence at, and upstream of, the transcription start site and, in yeast and human cells, by intracellular NAD⁺ and NADH levels. Our findings indicate mtRNAPs serve as both sensors and actuators in coupling cellular metabolism to mitochondrial transcriptional outputs, sensing NAD⁺ and NADH levels and adjusting transcriptional outputs accordingly.

Analysis of Bacterial Transcription by "Massively Systematic Transcript End Readout," MASTER

A systems-level view of cellular gene expression requires understanding the mechanistic principles governing each step of transcription. In this chapter, we describe a massively multiplexed method for the analysis of the relationship between nucleic acid sequence and transcription termed "MASTER," for massively systematic transcript end readout. MASTER enables parallel measurements of transcription output from at least 4¹⁰ (~1,000,000) individual template sequences in vitro and in vivo. MASTER involves constructing a DNA template library of barcoded sequences, generating RNA transcripts from the library during transcription in vitro or in vivo, and analyzing the relative abundance and 5'-end sequences of the RNA transcripts by high-throughput sequencing. MASTER provides a powerful, rapid, and versatile method to identify sequence determinants of each step of transcription and to define the mechanistic basis by which these sequence determinants dictate transcription output.

The RNA polymerase clamp interconverts dynamically among three states and is stabilized in a partly closed state by ppGpp

RNA polymerase (RNAP) contains a mobile structural module, the 'clamp,' that forms one wall of the RNAP active-center cleft and that has been linked to crucial aspects of the transcription cycle, including promoter melting, transcription elongation complex stability, transcription pausing, and transcription termination. Using single-molecule FRET on surface-immobilized RNAP molecules, we show that the clamp in RNAP holoenzyme populates three distinct conformational states and interconvert between these states on the 0.1-1 s time-scale. Similar studies confirm that the RNAP clamp is closed in open complex (RPO) and in initial transcribing complexes (RPITC), including paused initial transcribing complexes, and show that, in these complexes, the clamp does not exhibit dynamic behaviour. We also show that, the stringent-response alarmone ppGpp, which reprograms transcription during amino acid starvation stress, selectively stabilizes the partly-closed-clamp state and prevents clamp opening; these results raise the possibility that ppGpp controls promoter opening by modulating clamp dynamics.

Serum responsive proteome reveals correlation between oxidative phosphorylation and morphogenesis in Candida albicans ATCC10231

To understand the impact of fetal bovine serum (FBS) on metabolism and cellular architecture in addition to morphogenesis, we have identified FBS responsive proteome of Candida albicans. FBS induced 34% hyphae and 60% pseudohyphae in C. albicans at 30 °C while 98% hyphae at 37 °C. LC-MS/MS analysis revealed that 285 proteins modulated significantly in response to FBS at 30 °C and 37 °C. Out of which 152 were upregulated and 62 were downregulated at 30 °C while 18 were up and 53 were downregulated at 37 °C. Functional annotation suggests that FBS may inhibit glycolysis and fermentative pathway and enhance oxidative phosphorylation (OxPhos), TCA cycle, amino acid and fatty acid metabolism indicating a use of alternative energy source by C. albicans. OxPhos inhibition assay using sodium azide corroborated the correlation between inhibition of glycolysis and enhanced OxPhos with pseudohyphae formation. C. albicans induced hyphae in response to FBS irrespective of down regulation of Ras1,Asr1/Asr2, indicates the possible involvement of MAPK and cAMP-PKA independent pathway. The Cell wall of cells grown in presence of FBS at 30 °C was rich in mannan, Beta 1,3-glucan and chitin while membranes were rich in ergosterol compared to those grown at 37 °C.

SIGNIFICANCE OF THE STUDY

This is the first study suggesting a correlation between OxPhos and morphogenesis especially pseudohyphae formation in C. albicans. Our data also indicate that fetal bovine serum (FBS) induced morphogenesis is multifactorial and may involve MAPK and cAMP-PKA independent pathway. In addition to morphogenesis, our study provides an insight in to the modulation of metabolism and cellular architecture of C. albicans in response to FBS.

Structures of Bacterial RNA Polymerase Complexes Reveal the Mechanism of DNA Loading and Transcription Initiation

Gene transcription is carried out by multi-subunit RNA polymerases (RNAPs). Transcription initiation is a dynamic multi-step process that involves the opening of the double-stranded DNA to form a transcription bubble and delivery of the template strand deep into the RNAP for RNA synthesis. Applying cryoelectron microscopy to a unique transcription system using σ54 (σN), the major bacterial variant sigma factor, we capture a new intermediate state at 4.1 Å where promoter DNA is caught at the entrance of the RNAP cleft. Combining with new structures of the open promoter complex and an initial de novo transcribing complex at 3.4 and 3.7 Å, respectively, our studies reveal the dynamics of DNA loading and mechanism of transcription bubble stabilization that involves coordinated, large-scale conformational changes of the universally conserved features within RNAP and DNA. In addition, our studies reveal a novel mechanism of strand separation by σ54.

Association of ω with the C-Terminal Region of the β' Subunit Is Essential for Assembly of RNA Polymerase in Mycobacterium tuberculosis

The ω subunit is the smallest subunit of bacterial RNA polymerase (RNAP). Although homologs of ω are essential in both eukaryotes and archaea, this subunit has been known to be dispensable for RNAP in Escherichia coli and in other bacteria. In this study, we characterized an indispensable role of the ω subunit in Mycobacterium tuberculosis Unlike the well-studied E. coli RNAP, the M. tuberculosis RNAP core enzyme cannot be functionally assembled in the absence of the ω subunit. Importantly, substitution of M. tuberculosis ω with ω subunits from E. coli or Thermus thermophilus cannot restore the assembly of M. tuberculosis RNAP. Furthermore, by replacing different regions in M. tuberculosis ω with the corresponding regions from E. coli ω, we found a nonconserved loop region in M. tuberculosis ω essential for its function in RNAP assembly. From RNAP structures, we noticed that the location of the C-terminal region of the β' subunit (β'CTD) in M. tuberculosis RNAP but not in E. coli or T. thermophilus RNAP is close to the ω loop region. Deletion of this β'CTD in M. tuberculosis RNAP destabilized the binding of M. tuberculosis ω on RNAP and compromised M. tuberculosis core assembly, suggesting that these two regions may function together to play a role in ω-dependent RNAP assembly in M. tuberculosis Sequence alignment of the ω loop and the β'CTD regions suggests that the essential role of ω is probably restricted to mycobacteria. Together, our study characterized an essential role of M. tuberculosis ω and highlighted the importance of the ω loop region in M. tuberculosis RNAP assembly.IMPORTANCE DNA-dependent RNA polymerase (RNAP), which consists of a multisubunit core enzyme (α₂ββ'ω) and a dissociable σ subunit, is the only enzyme in charge of transcription in bacteria. As the smallest subunit, the roles of ω remain the least well studied. In Escherichia coli and some other bacteria, the ω subunit is known to be nonessential for RNAP. In this study, we revealed an essential role of the ω subunit for RNAP assembly in the human pathogen Mycobacterium tuberculosis, and a mycobacterium-specific ω loop that plays a role in this function was also characterized. Our study provides fresh insights for further characterizing the roles of bacterial ω subunit.

Structural basis of bacterial transcription activation

In bacteria, the activation of gene transcription at many promoters is simple and only involves a single activator. The cyclic adenosine 3',5'-monophosphate receptor protein (CAP), a classic activator, is able to activate transcription independently through two different mechanisms. Understanding the class I mechanism requires an intact transcription activation complex (TAC) structure at a high resolution. Here we report a high-resolution cryo-electron microscopy structure of an intact Escherichia coli class I TAC containing a CAP dimer, a σ⁷⁰-RNA polymerase (RNAP) holoenzyme, a complete class I CAP-dependent promoter DNA, and a de novo synthesized RNA oligonucleotide. The structure shows how CAP wraps the upstream DNA and how the interactions recruit RNAP. Our study provides a structural basis for understanding how activators activate transcription through the class I recruitment mechanism.

Identification of new antibacterial targets in RNA polymerase of Mycobacterium tuberculosis by detecting positive selection sites

Bacterial RNA polymerase (RNAP) is an effective target for antibacterial treatment. In order to search new potential targets in RNAP of Mycobacterium, we detected adaptive selections of RNAP related genes in 13 strains of Mycobacterium by phylogenetic analysis. We first collected sequences of 17 genes including rpoA, rpoB, rpoC, rpoZ, and sigma factor A-M. Then maximum likelihood trees were constructed, followed by positive selection detection. We found that sigG shows positive selection along the clade (M. tuberculosis, M. bovis), suggesting its important evolutionary role and its potential to be a new antibacterial target. Moreover, the regions near 933Cys and 935His on the rpoB subunit of M. tuberculosis showed significant positive selection, which could also be a new attractive target for anti-tuberculosis drugs.