The amino terminus of bacteriophage lambda integrase is involved in protein-protein interactions during recombination

Virus-associated organosulfur metabolism in human and environmental systems

Viruses influence the fate of nutrients and human health by killing microorganisms and altering metabolic processes. Organosulfur metabolism and biologically derived hydrogen sulfide play dynamic roles in manifestation of diseases, infrastructure degradation, and essential biological processes. Although microbial organosulfur metabolism is well studied, the role of viruses in organosulfur metabolism is unknown. Here, we report the discovery of 39 gene families involved in organosulfur metabolism encoded by 3,749 viruses from diverse ecosystems, including human microbiomes. The viruses infect organisms from all three domains of life. Six gene families encode for enzymes that degrade organosulfur compounds into sulfide, whereas others manipulate organosulfur compounds and may influence sulfide production. We show that viral metabolic genes encode key enzymatic domains, are translated into protein, and are maintained after recombination, and sulfide provides a fitness advantage to viruses. Our results reveal viruses as drivers of organosulfur metabolism with important implications for human and environmental health.

A novel λ integrase-mediated seamless vector transgenesis platform for therapeutic protein expression

Advances in stem cell engineering, gene therapy and molecular medicine often involve genome engineering at a cellular level. However, functionally large or multi transgene cassette insertion into the human genome still remains a challenge. Current practices such as random transgene integration or targeted endonuclease-based genome editing are suboptimal and might pose safety concerns. Taking this into consideration, we previously developed a transgenesis tool derived from phage λ integrase (Int) that precisely recombines large plasmid DNA into an endogenous sequence found in human Long INterspersed Elements-1 (LINE-1). Despite this advancement, biosafety concerns associated with bacterial components of plasmids, enhanced uptake and efficient transgene expression remained problematic. We therefore further improved and herein report a more superior Int-based transgenesis tool. This novel Int platform allows efficient and easy derivation of sufficient amounts of seamless supercoiled transgene vectors from conventional plasmids via intramolecular recombination as well as subsequent intermolecular site-specific genome integration into LINE-1. Furthermore, we identified certain LINE-1 as preferred insertion sites for Int-mediated seamless vector transgenesis, and showed that targeted anti-CD19 chimeric antigen receptor gene integration achieves high-level sustained transgene expression in human embryonic stem cell clones for potential downstream therapeutic applications.

Protein binding sites involved in the assembly of the KplE1 prophage intasome

The organization of the recombination regions of the KplE1 prophage in Escherichia coli K12 differs from that observed in the lambda prophage. Indeed, the binding sites characterized for the IntS integrase, the TorI recombination directionality factor (RDF) and the integration host factor (IHF) vary in number, spacing and orientation on the attL and attR regions. In this paper, we performed site-directed mutagenesis of the recombination sites to decipher if all sites are essential for the site-specific recombination reaction and how the KplE1 intasome is assembled. We also show that TorI and IntS form oligomers that are stabilized in the presence of their target DNA. Moreover, we found that IHF is the only nucleoid associated protein (NAP) involved in KplE1 recombination, although it is dispensable. This is consistent with the presence of only one functional IHF site on attR and none on attL.

Mutations associated with failure of raltegravir treatment affect integrase sensitivity to the inhibitor in vitro

Raltegravir (MK-0518) is a potent inhibitor of human immunodeficiency virus (HIV) integrase and is clinically effective against viruses resistant to other classes of antiretroviral agents. However, it can select mutations in the HIV integrase gene. Nine heavily pretreated patients who received salvage therapy including raltegravir and who subsequently developed virological failure under raltegravir therapy were studied. For each patient, the sequences of the integrase-coding region were determined and compared to that at the beginning of the treatment. Four different mutation profiles were identified in these nine patients: E92Q, G140S Q148H, N155H, and E157Q mutations. For four patients, each harboring a different profile, the wild-type and mutated integrases were produced, purified, and assayed in vitro. All the mutations identified altered the activities of integrase protein: both 3' processing and strand transfer activities were moderately affected in the E92Q mutant; strand transfer was markedly impaired in the N155H mutant; both activities were strongly impaired in the G140S Q148H mutant; and the E157Q mutant was almost completely inactive. The sensitivities of wild-type and mutant integrases to raltegravir were compared. The E92Q and G140S Q148H profiles were each associated with a 7- to 8-fold decrease in sensitivity, and the N155H mutant was more than 14-fold less sensitive to raltegravir. At least four genetic profiles (E92Q, G140S Q148H, N155H, and E157Q) can be associated with in vivo treatment failure and resistance to raltegravir. These mutations led to strong impairment of enzymes in vitro in the absence of raltegravir: strand transfer activity was affected, and in some cases 3' processing was also impaired.

Mutations in the amino-terminal domain of lambda-integrase have differential effects on integrative and excisive recombination

Lambda integrase (Int) forms higher-order protein-DNA complexes necessary for site-specific recombination. The carboxy-terminal domain of Int (75-356) is responsible for catalysis at specific core-type binding sites whereas the amino-terminal domain (1-70) is responsible for cooperative arm-type DNA binding. Alanine scanning mutagenesis of residues 64-70, within full-length integrase, has revealed differential effects on cooperative arm binding interactions that are required for integrative and excisive recombination. Interestingly, while these residues are required for cooperative arm-type binding on both P'1,2 and P'2,3 substrates, cooperative binding at the arm-type sites P'2,3 was more severely compromised than binding at arm-type sites P'1,2 for L64A. Concomitantly, L64A had a much stronger effect on integrative than on excisive recombination. The arm-binding properties of Int appear to be intrinsic to the amino-terminal domain because the phenotype of L64A was the same in an amino-terminal fragment (Int 1-75) as it was in the full-length protein.

Dimerization of bacteriophage P2 integrase is not required for binding to its DNA target but for its biological activity

Coliphage P2 integrates into the host chromosome upon lysogenization via site-specific recombination mediated by the phage integrase (Int). P2 integrase belongs to the tyrosine family of recombinases. In this work, it is shown that P2 integrase forms dimers but not oligomers in the absence of its DNA target. Furthermore, the C-terminal end of the protein and amino acid (aa) E197 have been found to be involved in dimerization. Amino acid E197 is located in a conserved region of the tyrosine recombinases that has not previously been implicated in dimerization. The dimerization deficient mutants were unaffected in binding to its phage attachment site (attP) substrate, but had a reduced ability to complement an int-defective prophage.

Two structural features of lambda integrase that are critical for DNA cleavage by multimers but not by monomers

Despite many years of genetic and biochemical studies on the lambda integrase (Int) recombination system, it is still not known whether the Int protein is competent for DNA cleavage as a monomer. We have addressed this question, as part of a larger study of Int functions critical for the formation of higher-order complexes, by isolating "multimer-specific" mutants. We identify a pair of oppositely charged residues, E153 and R169, that comprise an intermolecular salt bridge within a functional Int multimer. Mutation of either of these residues significantly reduces both the cleavage of full-att sites and the resolution of Holliday junctions without compromising the cleavage of half-att site substrates. Allele-specific suppressor mutations were generated at these residues. Their interaction with wild-type Int on preformed Holliday junctions indicates that the mutated residues comprise an intermolecular salt bridge. We have also shown that the most C-terminal seven residues of Int, which comprise another previously identified subunit interface, inhibit DNA cleavage by monomeric but not multimeric Int. Taken together, our results lead us to conclude that Int can cleave DNA as a monomer. We also identify and discuss unique structural features of Int that act negatively to reduce its activity as a monomer and other features that act positively to enhance its activity as a multimer.

The molecular basis of co-operative DNA binding between lambda integrase and excisionase

Higher-order nucleoprotein complexes often stabilize catalytic proteins in appropriate conformations for optimal activity and contribute to regulation during reactions requiring association of proteins and DNA. Formation of such complexes, known as intasomes, is required for site-specific recombination catalysed by bacteriophage Lambda Integrase protein (Int). Int-catalysed recombination is regulated by a second bacteriophage-encoded protein, Excisionase (Xis), which both stimulates excision and inhibits integration. To exert its effect, Xis binds co-operatively with Int, thereby inducing and stabilizing a DNA bend that alters the intasome structures formed during recombination. A rare int mutant, int 2268 ts, was reported (Enquist, L.W. and Weisberg, R.A. (1984) Mol Gen Genet 195: 62-69) to be more defective for excision than integration. Here, we have determined that this mutant Int protein contains an E47K substitution, and that the resultant excision-specific defect is due, at least in part, to destabilized interactions between Int and Xis. Analysis of several engineered substitutions at Int position 47 showed that a negatively charged residue is required for co-operative DNA binding between Int and Xis, and suggest that the Int-E47 residue may contact Xis directly. Substitutions at Int position 47 also affect co-operative binding among Int proteins at arm-type DNA sites, and thereby reduce the efficiency of both integration and excision. Collectively, these results suggest that a single surface of the Int amino-terminal domain mediates two alternate types of co-operative binding interactions.

Mutations at residues 282, 286, and 293 of phage lambda integrase exert pathway-specific effects on synapsis and catalysis in recombination

Bacteriophage lambda integrase (Int) catalyzes site-specific recombination between pairs of attachment (att) sites. The att sites contain weak Int-binding sites called core-type sites that are separated by a 7-bp overlap region, where cleavage and strand exchange occur. We have characterized a number of mutant Int proteins with substitutions at positions S282 (S282A, S282F, and S282T), S286 (S286A, S286L, and S286T), and R293 (R293E, R293K, and R293Q). We investigated the core- and arm-binding properties and cooperativity of the mutant proteins, their ability to catalyze cleavage, and their ability to form and resolve Holliday junctions. Our kinetic analyses have identified synapsis as the rate-limiting step in excisive recombination. The IntS282 and IntS286 mutants show defects in synapsis in the bent-L and excisive pathways, respectively, while the IntR293 mutants exhibit synapsis defects in both the excision and bent-L pathways. The results of our study support earlier findings that the catalytic domain also serves a role in binding to core-type sites, that the core contacts made by this domain are important for both synapsis and catalysis, and that Int contacts core-type sites differently among the four recombination pathways. We speculate that these residues are important for the proper positioning of the catalytic residues involved in the recombination reaction and that their positions differ in the distinct nucleoprotein architectures formed during each pathway. Finally, we found that not all catalytic events in excision follow synapsis: the attL site probably undergoes several rounds of cleavage and ligation before it synapses and exchanges DNA with attR.

Control of directionality in L5 integrase-mediated site-specific recombination

Mycobacteriophage L5 is a temperate phage that forms lysogens in Mycobacterium smegmatis. These lysogens carry an integrated L5 prophage inserted at a specific chromosomal location and undergo subsequent excision during induction of lytic growth. Both the integrative and excisive site-specific recombination events are catalyzed by the phage-encoded tyrosine integrase (Int-L5) and require the host-encoded protein, mIHF. The directionality of these recombination events is determined by a second phage-encoded protein, Excise, the product of gene 36 (Xis-L5); integration occurs efficiently in the absence of Xis-L5 while excision is dependent upon it. We show here that Xis-L5 binds to attR DNA, introduces a DNA bend, and facilitates the formation of an intasome-R complex. This complex, which requires mIHF, Xis-L5 and Int-L5, readily recombines with a second intasome formed by Int-L5, mIHF and attL DNA (intasome-L) to generate the attP and attB products of excision. Xis-L5 also strongly inhibits Int-L5-mediated integrative recombination but does not prevent either the protein-DNA interactions that form the attP intasome (intasome-P) or the capture of attB, but acts later in the reaction presumably by preventing the formation of a recombinagenic synaptic intermediate. The mechanism of action of Xis-L5 appears to be purely architectural, influencing the assembly of protein-DNA structures solely through its DNA-binding and DNA-bending properties.

Differential affinity and cooperativity functions of the amino-terminal 70 residues of lambda integrase

The site-specific recombinase (Int) of bacteriophage lambda is a heterobivalent DNA-binding protein that binds two different classes of DNA-binding sites within its recombination target sites. The several functions of Int are apportioned between a large carboxy-terminal domain that cleaves and ligates DNA at each of its four "core-type" DNA-binding sites and a small amino-terminal domain, whose primary function is binding to each of its five "arm-type" DNA sites, which are distant from the core region. Int bridges between the two classes of binding sites are facilitated by accessory DNA-bending proteins that along with Int comprise higher-order recombinogenic complexes. We show here that although the 64 amino-terminal residues of Int bind efficiently to a single arm site, this protein cannot form doubly bound complexes on adjacent arm sites. However, 1-70 Int does show the same cooperative binding to adjacent arm sites as the full length protein. We also found that 1-70 Int specifies cooperative interactions with the accessory protein Xis when the two are bound to their adjacent cognate sites P2 and X1, respectively. To complement the finding that these two amino-terminal domain functions (along with arm DNA binding) are all specified by residues 1-70, we determined that Thr75 is the first residue of the minimal carboxy-terminal domain, thereby identifying a specific interdomain linker region. We have measured the affinity constants for Int binding to each of the five arm sites and the cooperativity factors for Int binding to the two pairs of adjacent arm sites, and we have identified several DNA structural features that contribute to the observed patterns of Int binding to arm sites. Taken together, the results highlight several interesting features of arm DNA binding that invite speculation about additional levels of complexity in the regulation of lambda site-specific recombination.

Regulation of site-specific recombination by the C-terminus of lambda integrase

Site-specific recombination catalyzed by bacteriophage lambda integrase (Int) is essential for establishment and termination of the viral lysogenic life cycle. Int is the archetype of the tyrosine recombinase family whose members are responsible for DNA rearrangement in prokaryotes, eukaryotes and viruses. The mechanism regulating catalytic activity during recombination is incompletely understood. Studies of tyrosine recombinases bound to their target substrates suggest that the C-termini of the proteins are involved in protein-protein contacts that control the timing of DNA cleavage events during recombination. We investigated an Int truncation mutant (W350) that possesses enhanced topoisomerase activity but greater than 100-fold reduced recombination activity. Alanine scanning mutagenesis of the C-terminus indicates that two mutants, W350A and I353A, cannot perform site-specific recombination although their DNA binding, cleavage and ligation activities are at wild-type levels. Two other mutants, R346A and R348A, are deficient solely in the ability to cleave DNA. To explain these results, we have constructed a homology-threaded model of the Int structure using a Cre crystal structure. We propose that residues R346 and R348 are involved in orientation of the catalytic tyrosine that cleaves DNA, whereas W350 and I353 control and make intermolecular contacts with other Int proteins in the higher order recombination structures known as intasomes. These results suggest that Int and the other tyrosine recombinases have evolved regulatory contacts that coordinate site-specific recombination at the C-terminus.

Binding of the baculovirus very late expression factor 1 (VLF-1) to different DNA structures

BACKGROUND

Baculovirus genomes encode a gene called very late expression factor 1 (VLF-1) that is a member of the integrase (Int) family of proteins. In this report we describe the binding properties of purified Autographa californica multiple capsid nucleopolyhedrovirus (AcMNPV) VLF-1 to a number of different DNA structures including homologous regions. In addition, its enzymatic activity was examined.

RESULTS

VLF-1 was expressed in a recombinant baculovirus as a fusion with both HA and HIS6 tags and its binding activity to different DNA structures was tested. No binding was evident to single and double strand structures, very low binding was observed to Y-forks, more binding was observed to three-way junctions, whereas cruciform structures showed high levels of binding. VLF-1 binding was affected by divalent cations; optimal binding to three-way junctions and cruciforms was 2 and 0 mM MgCl2, respectively. Homologous region (hr) sequences was also examined including oligomers designed to expose the hr palindrome as a hairpin, linear double strand, or H-shaped structure. Efficient binding was observed to the hairpin and H-shaped structure. No topoisomerase or endonuclease activity was detected. Sedimentation analysis indicated that *VLF-1 is present as a monomer.

CONCLUSIONS

An HA- and HIS-tagged version of AcMNPV VLF-1 showed structure-dependent binding to DNA substrates with the highest binding affinity to cruciform DNA. These results are consistent with the involvement of VLF-1 in the processing of branched DNA molecules at the late stages of viral genome replication. We were unable to detect enzymatic activity associated with these complexes.

Interactions between integrase and excisionase in the phage lambda excisive nucleoprotein complex

Bacteriophage lambda site-specific recombination comprises two overall reactions, integration into and excision from the host chromosome. Lambda integrase (Int) carries out both reactions. During excision, excisionase (Xis) helps Int to bind DNA and introduces a bend in the DNA that facilitates formation of the proper excisive nucleoprotein complex. The carboxyl-terminal alpha-helix of Xis is thought to interact with Int through direct protein-protein interactions. In this study, we used gel mobility shift assays to show that the amino-terminal domain of Int maintained cooperative interactions with Xis. This finding indicates that the amino-terminal arm-type DNA binding domain of Int interacts with Xis.

Site-specific recombination in eukaryotic cells mediated by mutant lambda integrases: implications for synaptic complex formation and the reactivity of episomal DNA segments

Mutant lambda integrases catalyze site-specific DNA recombination in the absence of accessory factors IHF, XIS, and negative DNA supercoiling. Here we investigate the effects that a human cellular environment exerts on these reactions in order to (i) gain further insights into mechanistic aspects of recombination in eukaryotic cells and (ii) to further develop the Int system for biotechnological applications. First, we compared intra- and intermolecular integrative as well as excisive recombination pathways on episomal substrates after co-transfection with recombinase expression vectors. Our results demonstrate that, within 24 hours after transfection, intermolecular recombination by mutant integrase is at least as efficient as intramolecular recombination. Second, a significant intermolecular recombination activity was observed between two copies of a recombination site containing only the 21 bp comprising core-type DNA sequence. This basic activity was stimulated several-fold when arm-type DNA sequences were present in addition to core sites. Therefore, one recombination pathway in human cells involves mutant integrases bound solely at core sites, which is reminiscent of the Flp/FRT and Cre/loxP pathways. The stimulatory effect of arm-type sequences could be explained by an increase in integrase concentration in the vicinity of core sites. We show, in addition, that an N-terminal truncated mutant integrase exhibited only a very weak recombinogenic activity in a eukaryotic background. This result strengthens a functional role for the N-terminal domain in recombination in addition to its arm-type DNA-binding activity. Finally, we demonstrate that low level integrative recombination by wild-type integrase is stimulated when purified integration host factor is co-transfected. This corroborates our previous conclusion that sufficient amounts of eukaryotic protein co-factors, which could functionally replace IHF, are not present in human cells. It also provides a potential means to control site-specific recombination in eukaryotic cells.

Arm-site binding by lambda -integrase: solution structure and functional characterization of its amino-terminal domain

The integrase protein (Int) from bacteriophage lambda catalyzes the insertion and excision of the viral genome into and out of Escherichia coli. It is a member of the lambda-Int family of site-specific recombinases that catalyze a diverse array of DNA rearrangements in archaebacteria, eubacteria, and yeast and belongs to the subset of this family that possesses two autonomous DNA-binding domains. The heterobivalent properties of Int can be decomposed into a carboxyl-terminal domain that executes the DNA cleavage and ligation reactions and a smaller amino-terminal domain that binds to an array of conserved DNA sites within the phage arms, thereby arranging Int protomers within the higher-order recombinogenic complex. We have determined that residues Met-1 to Leu-64 of Int constitute the minimal arm-type DNA-binding domain (INT-DBD(1-64)) and solved the solution structure by using NMR. We show that the INT-DBD(1-64) is a novel member of the growing family of three-stranded beta-sheet DNA-binding proteins, because it supplements this motif with a disordered amino-terminal basic tail that is important for arm-site binding. A model of the arm-DNA-binding domain recognizing its cognate DNA site is proposed on the basis of similarities with the analogous domain of Tn916 Int and is discussed in relation to other features of the protein.

The small DNA binding domain of lambda integrase is a context-sensitive modulator of recombinase functions

lambda Integrase (Int) has the distinctive ability to bridge two different and well separated DNA sequences. This heterobivalent DNA binding is facilitated by accessory DNA bending proteins that bring flanking Int sites into proximity. The regulation of lambda recombination has long been perceived as a structural phenomenon based upon the accessory protein-dependent Int bridges between high-affinity arm-type (bound by the small N-terminal domain) and low-affinity core-type DNA sites (bound by the large C-terminal domain). We show here that the N-terminal domain is not merely a guide for the proper positioning of Int protomers, but is also a context-sensitive modulator of recombinase functions. In full-length Int, it inhibits C-terminal domain binding and cleavage at the core sites. Surprisingly, its presence as a separate molecule stimulates the C-terminal domain functions. The inhibition in full-length Int is reversed or overcome in the presence of arm-type oligonucleotides, which form specific complexes with Int and core-type DNA. We consider how these results might influence models and experiments pertaining to the large family of heterobivalent recombinases.

Characterization of a mutation of bacteriophage lambda integrase. Putative role in core binding and strand exchange for a conserved residue

Site-specific recombination is involved in processes ranging from resolution of bacterial chromosome dimers to adeno-associated viral integration and is a versatile tool for mammalian genetics. The bacteriophage lambda-encoded site-specific recombinase integrase (Int) is one of the best studied site-specific recombinases and mediates recombination via four distinct pathways. We have characterized a mutant version of lambda Int, IntT236I; this mutant can perform the bent-L pathway only, whereas the corresponding IntT236A mutant can perform bent-L, excision and integration pathways. Experiments with both IntT236I and IntT236A show that the hydroxyl group of threonine is necessary for wild-type recombination. Substitution of the threonine by serine leads to nearly complete rescue of the mutant phenotypes. In addition, our data show that the IntT236I mutant is defective partially due to obstructive steric interactions. Comparisons of crystal structures reveal that the threonine at residue 236 may play an important role in stabilizing recombination intermediates through solvent-mediated protein-DNA interactions at the core-binding sites and that the hydroxyl group is important for effective cleavage and Holliday junction formation. Our data also indicate that Int contacts the core sites differently in intermediates assembled in excisive versus bent-L recombination.