Arm sequences contribute to the architecture and catalytic function of a lambda integrase-Holliday junction complex

Resolution of Mismatched Overlap Holliday Junction Intermediates by the Tyrosine Recombinase IntDOT

CTnDOT is an integrated conjugative element found in Bacteroides species. CTnDOT contains and transfers antibiotic resistance genes. The element integrates into and excises from the host chromosome via a Holliday junction (HJ) intermediate as part of a site-specific recombination mechanism. The CTnDOT integrase, IntDOT, is a tyrosine recombinase with core-binding, catalytic, and amino-terminal (N) domains. Unlike well-studied tyrosine recombinases, such as lambda integrase (Int), IntDOT is able to resolve Holliday junctions containing heterology (mismatched bases) between the sites of strand exchange. All known natural isolates of CTnDOT contain mismatches in the overlap region between the sites of strand exchange. Previous work showed that IntDOT was unable to resolve synthetic Holliday junctions containing mismatched bases to products in the absence of the arm-type sites and a DNA-bending protein. We constructed synthetic HJs with the arm-type sites and tested them with the Bacteroides host factor (BHFa). We found that the addition of BHFa stimulated resolution of HJ intermediates with mismatched overlap regions to products. In addition, the L1 site is required for directionality of the reaction, particularly when the HJ contains mismatches. BHFa is required for product formation when the overlap region contains mismatches, and it stimulates resolution to products when the overlap region is identical. Without this DNA bending, the N domain of IntDOT is likely unable to bind the L1 arm-type site. These findings suggest that BHFa bends DNA into the necessary conformation for the higher-order complexes, including the L1 site, that are required for product formation.

IMPORTANCE CTnDOT is a mobile element that carries antibiotic resistance genes and moves by site-selective recombination and subsequent conjugation. The recombination reaction is catalyzed by an integrase IntDOT that is a member of the tyrosine recombinase family. The reaction proceeds through ordered strand exchanges that generate a Holliday junction (HJ) intermediate. Unlike other tyrosine recombinases, IntDOT can resolve HJs containing mismatched bases in the overlap region in vivo , as is the case under natural conditions. However, HJ intermediates including only IntDOT core-type sites cannot be resolved to products if the HJ intermediate contains mismatched bases. We added arm-type sites in cis and in trans to the HJ intermediates and the protein BHFa to study the requirements for higher-order nucleoprotein complexes.

CTnDOT integrase interactions with attachment site DNA and control of directionality of the recombination reaction

IntDOT is a tyrosine recombinase encoded by the conjugative transposon CTnDOT. The core binding (CB) and catalytic (CAT) domains of IntDOT interact with core-type sites adjacent to the regions of strand exchange, while the N-terminal arm binding (N) domain interacts with arm-type sites distal to the core. Previous footprinting experiments identified five arm-type sites, but how the arm-type sites participate in the integration and excision of CTnDOT was not known. In vitro integration assays with substrates containing arm-type site mutants demonstrated that attDOT sequences containing mutations in the L1 arm-type site or in the R1 and R2 or R1 and R2' arm-type sites were dramatically defective in integration. Substrates containing mutations in the L1 and R1 arm-type sites showed a 10- to 20-fold decrease in detectable in vitro excision, but introduction of multiple arm-type site mutations in attR did not have an effect on the excision frequency. A sixth arm-type site, the R1' site, was also identified and shown to be required for integration and important for efficient excision. These results suggest that intramolecular IntDOT interactions are required for integration, while the actions of accessory factors are more important for excision. Gel shift assays performed in the presence of core- and arm-type site DNAs showed that IntDOT affinity for the attDOT core was enhanced when IntDOT was simultaneously bound to arm-type site DNA.

Control of directionality in the DNA strand-exchange reaction catalysed by the tyrosine recombinase TnpI

In DNA site-specific recombination catalysed by tyrosine recombinases, two pairs of DNA strands are sequentially exchanged between separate duplexes and the mechanisms that confer directionality to this theoretically reversible reaction remain unclear. The tyrosine recombinase TnpI acts at the internal resolution site (IRS) of the transposon Tn4430 to resolve intermolecular transposition products. Recombination is catalysed at the IRS core sites (IR1–IR2) and is regulated by adjacent TnpI-binding motifs (DR1 and DR2). These are dispensable accessory sequences that confer resolution selectivity to the reaction by stimulating synapsis between directly repeated IRSs. Here, we show that formation of the DR1–DR2-containing synapse imposes a specific order of activation of the TnpI catalytic subunits in the complex so that the IR1-bound subunits catalyse the first strand exchange and the IR2-bound subunits the second strand exchange. This ordered pathway was demonstrated for a complete recombination reaction using a TnpI catalytic mutant (TnpI-H234L) partially defective in DNA rejoining. The presence of the DR1- and DR2-bound TnpI subunits was also found to stabilize transient recombination intermediates, further displacing the reaction equilibrium towards product formation. Implication of TnpI/IRS accessory elements in the initial architecture of the synapse and subsequent conformational changes taking place during strand exchange is discussed.

Selection of bacteriophage lambda integrases with altered recombination specificity by in vitro compartmentalization

In vitro compartmentalization (IVC) was employed for the first time to select for novel bacteriophage λ integrase variants displaying significantly enhanced recombination activity on a non-cognate target DNA sequence. These variants displayed up to 9-fold increased recombination activity over the parental enzyme, and one mutant recombined the chosen non-cognate substrate more efficiently than the parental enzyme recombined the wild-type DNA substrate. The in vitro specificity phenotype extended to the intracellular recombination of episomal vectors in HEK293 cells. Surprisingly, mutations conferring the strongest phenotype do not occur in the λ integrase core-binding domain, which is known to interact directly with cognate target sequences. Instead, they locate to the N-terminal domain which allosterically modulates integrase activity, highlighting a previously unknown role for this domain in directing integrase specificity. The method we describe provides a robust, completely in vitro platform for the development of novel integrase reagent tools for in vitro DNA manipulation and other biotechnological applications.

NMR structure of the amino-terminal domain of the lambda integrase protein in complex with DNA: immobilization of a flexible tail facilitates beta-sheet recognition of the major groove

The integrase protein (Int) from bacteriophage lambda is the archetypal member of the tyrosine recombinase family, a large group of enzymes that rearrange DNA in all domains of life. Int catalyzes the insertion and excision of the viral genome into and out of the Escherichia coli chromosome. Recombination transpires within higher-order nucleoprotein complexes that form when its amino-terminal domain binds to arm-type DNA sequences that are located distal to the site of strand exchange. Arm-site binding by Int is essential for catalysis, as it promotes Int-mediated bridge structures that stabilize the recombination machinery. We have elucidated how Int is able to sequence specifically recognize the arm-type site sequence by determining the solution structure of its amino-terminal domain (Int(N), residues Met1 to Leu64) in complex with its P'2 DNA binding site. Previous studies have shown that Int(N) adopts a rare monomeric DNA binding fold that consists of a three-stranded antiparallel beta-sheet that is packed against a carboxy-terminal alpha helix. A low-resolution crystal structure of the full-length protein also revealed that the sheet is inserted into the major groove of the arm-type site. The solution structure presented here reveals how Int(N) specifically recognizes the arm-type site sequence. A novel feature of the new solution structure is the use of an 11-residue tail that is located at the amino terminus. DNA binding induces the folding of a 3(10) helix in the tail that projects the amino terminus of the protein deep into the minor groove for stabilizing DNA contacts. This finding reveals the structural basis for the observation that the "unstructured" amino terminus is required for recombination.

Amino-terminal domain interactions of lambda integrase on arm-type DNA

In contrast to the other tyrosine recombinase family members, integrase protein (Int) of bacteriophage lambda has an additional amino-terminal domain that binds to "arm-type" DNA sequences distant from those involved in strand exchange. The homomeric interaction between neighboring amino-terminal domains of Int is contributed by R30-D71 salt-bridge in a non-equivalent manner on Holliday-junction intermediates. In this report, R30 and D71 residues were investigated in regard to Int's cooperative binding to "arm-type" DNA and the attenuating function of "arm-type" DNA. The results suggest the electrostatic interaction between residues 30 and 71 is dependent on "arm-type" DNA and contributes the "selective" inhibition of catalytic activity of lambda Int by "arm-type" DNA.

A biotin interference assay highlights two different asymmetric interaction profiles for lambda integrase arm-type binding sites in integrative versus excisive recombination

The site-specific recombinase integrase encoded by bacteriophage lambda promotes integration and excision of the viral chromosome into and out of its Escherichia coli host chromosome through a Holliday junction recombination intermediate. This intermediate contains an integrase tetramer bound via its catalytic carboxyl-terminal domains to the four "core-type" sites of the Holliday junction DNA and via its amino-terminal domains to distal "arm-type" sites. The two classes of integrase binding sites are brought into close proximity by an ensemble of accessory proteins that bind and bend the intervening DNA. We have used a biotin interference assay that probes the requirement for major groove protein binding at specified DNA loci in conjunction with DNA protection, gel mobility shift, and genetic experiments to test several predictions of the models derived from the x-ray crystal structures of minimized and symmetrized surrogates of recombination intermediates lacking the accessory proteins and their cognate DNA targets. Our data do not support the predictions of "non-canonical" DNA targets for the N-domain of integrase, and they indicate that the complexes used for x-ray crystallography are more appropriate for modeling excisive rather than integrative recombination intermediates. We suggest that the difference in the asymmetric interaction profiles of the N-domains and arm-type sites in integrative versus excisive recombinogenic complexes reflects the regulation of recombination, whereas the asymmetry of these patterns within each reaction contributes to directionality.

Trans cooperativity by a split DNA recombinase: the central and catalytic domains of bacteriophage lambda integrase cooperate in cleaving DNA substrates when the two domains are not covalently linked

Site-specific recombinases of the lambda-integrase family recognize and cleave their cognate DNA sites through cooperative binding to opposite sides of the DNA substrate by a C-terminal catalytic domain and a flexibly linked "core-binding" domain; regulation of this cleavage is achieved via the formation of higher-order complexes. We report that the core-binding domain of lambda-integrase is able to stimulate the activity of the catalytic domain even when the two domains are not linked. This trans stimulation is accomplished without significantly increasing the affinity of the catalytic domain for its DNA substrate. Moreover, we show that mutations in the DNA substrate can abrogate this effect while retaining specificity determinants for cleavage. Since the domains do not significantly interact directly, this finding implies that trans activation is achieved via the DNA substrate in a manner that may be mechanistically important in this and similar DNA binding and cleaving enzymes.

Mechanisms of site-specific recombination

Integration, excision, and inversion of defined DNA segments commonly occur through site-specific recombination, a process of DNA breakage and reunion that requires no DNA synthesis or high-energy cofactor. Virtually all identified site-specific recombinases fall into one of just two families, the tyrosine recombinases and the serine recombinases, named after the amino acid residue that forms a covalent protein-DNA linkage in the reaction intermediate. Their recombination mechanisms are distinctly different. Tyrosine recombinases break and rejoin single strands in pairs to form a Holliday junction intermediate. By contrast, serine recombinases cut all strands in advance of strand exchange and religation. Many natural systems of site-specific recombination impose sophisticated regulatory mechanisms on the basic recombinational process to favor one particular outcome of recombination over another (for example, excision over inversion or deletion). Details of the site-specific recombination processes have been revealed by recent structural and biochemical studies of members of both families.

Expression of phage P4 integrase is regulated negatively by both Int and Vis

Phage P4 int gene encodes the integrase responsible for phage integration into and excision from the Escherichia coli chromosome. Here, the data showing that P4 int expression is regulated in a complex manner at different levels are presented. First of all, the Pint promoter is regulated negatively by both Int and Vis, the P4 excisionase. The N-terminal portion of Int appears to be sufficient for such a negative autoregulation, suggesting that the Int N terminus is implicated in DNA binding. Second, full-length transcripts covering the entire int gene could be detected only upon P4 infection, whereas in P4 lysogens only short 5′-end covering transcripts were detectable. On the other hand, transcripts covering the 5′-end of int were also very abundant upon infection. It thus appears that premature transcription termination and/or mRNA degradation play a role in Int-negative regulation both on the basal prophage transcription and upon infection. Finally, comparison between Pint–lacZ transcriptional and translational fusions suggests that Vis regulates Int expression post-transcriptionally. The findings that Vis is also an RNA-binding protein and that Int may be translated from two different start codons have implications on possible regulation models of Int expression.

Lambda integrase: armed for recombination

Bacteriophage lambda moves its viral genome into and out of the bacterial chromosome using site-specific recombination. Crystal structures of reaction intermediates in this recombination pathway provide exciting new snapshots of full length lambda integrase interacting with both core and regulatory DNA elements.

DNA arms do the legwork to ensure the directionality of lambda site-specific recombination

The integrase protein of bacteriophage lambda (Int) catalyzes site-specific recombination between lambda phage and Escherichia coli genomes. Int is a tyrosine recombinase that binds to DNA core sites via a C-terminal catalytic domain and to a collection of arm DNA sites, distant from the site of recombination, via its N-terminal domain. The arm sites, in conjunction with accessory DNA-bending proteins, provide a means of regulating the efficiency and directionality of Int-catalyzed recombination. Recent crystal structures of lambda Int tetramers bound to synaptic and Holliday junction intermediates, together with new biochemical data, suggest a mechanism for the allosteric control of the recombination reaction through arm DNA binding interactions.

Receipt of the C-terminal tail from a neighboring lambda Int protomer allosterically stimulates Holliday junction resolution

Bacteriophage lambda integrase (Int) catalyzes the integration and excision of the phage lambda chromosome into and out of the Esherichia coli host chromosome. The seven carboxy-terminal residues (C-terminal tail) of Int comprise a context-sensitive regulatory element that links catalytic function with protein multimerization and also coordinates Int functions within the multimeric recombinogenic complex. The experiments reported here show that the beta5-strand of Int is not simply a placeholder for the C-terminal tail but rather exerts its own allosteric effects on Int function in response to the incoming tail. Using a mutant integrase in which the C-terminal tail has been deleted (W350ter), we demonstrate that the C-terminal tail is required for efficient and accurate resolution of Holliday junctions by tetrameric Int. Addition of a free heptameric peptide of the same sequence as the C-terminal tail partially reverses the W350ter defects by stimulating Holliday junction resolution. The peptide also stimulates the topoisomerase function of monomeric W350ter. Single residue alterations in the peptide sequence and a mutant of the beta5 strand indicate that the observed stimulation arises from specific contacts with the beta5 strand (residues 239-243). The peptide does not stimulate binding of W350ter to its cognate DNA sites and therefore appears to recapitulate the effects of the normal C-terminal tail intermolecular contacts in wild-type Int. Models for the allosteric stimulation of Int activity by beta5 strand contacts are discussed.

A structural basis for allosteric control of DNA recombination by lambda integrase

Site-specific DNA recombination is important for basic cellular functions including viral integration, control of gene expression, production of genetic diversity and segregation of newly replicated chromosomes, and is used by bacteriophage lambda to integrate or excise its genome into and out of the host chromosome. lambda recombination is carried out by the bacteriophage-encoded integrase protein (lambda-int) together with accessory DNA sites and associated bending proteins that allow regulation in response to cell physiology. Here we report the crystal structures of lambda-int in higher-order complexes with substrates and regulatory DNAs representing different intermediates along the reaction pathway. The structures show how the simultaneous binding of two separate domains of lambda-int to DNA facilitates synapsis and can specify the order of DNA strand cleavage and exchange. An intertwined layer of amino-terminal domains bound to accessory (arm) DNAs shapes the recombination complex in a way that suggests how arm binding shifts the reaction equilibrium in favour of recombinant products.

Origin pairing ('handcuffing') and unpairing in the control of P1 plasmid replication

The P1 plasmid origin has an array of five binding sites (iterons) for the plasmid-encoded initiator protein RepA. Saturation of these sites is required for initiation. Iterons can also pair via their bound RepAs. The reaction, called handcuffing, is believed to be the key to control initiation negatively. Here we have determined some of the mechanistic details of the reaction. We show that handcuffed RepA-iteron complexes dissociate when they are diluted or challenged with cold competitor iterons, suggesting spontaneous reversibility of the handcuffing reaction. The complex formation increases with increased RepA binding, but decreases upon saturation of binding. Complex formation also decreases in the presence of molecular chaperones (DnaK and DnaJ) that convert RepA dimers to monomers. This indicates that dimers participate in handcuffing, and that chaperones are involved in reversing handcuffing. They could play a direct role by reducing dimers and an indirect role by increasing monomers that would compete out the weaker binding dimers from the origin. We propose that an increased monomer to dimer ratio is the key to reverse handcuffing.

Cyclic changes in the affinity of protein-DNA interactions drive the progression and regulate the outcome of the Tn10 transposition reaction

The Tn10 transpososome is a DNA processing machine in which two transposon ends, a transposase dimer and the host protein integration host factor (IHF), are united in an asymmetrical complex. The transitions that occur during one transposition cycle are not limited to chemical cleavage events at the transposon ends, but also involve a reorganization of the protein and DNA components. Here, we demonstrate multiple pathways for Tn10 transposition. We show that one series of events is favored over all others and involves cyclic changes in the affinity of IHF for its binding site. During transpososome assembly, IHF is bound with high affinity. However, the affinity for IHF drops dramatically after cleavage of the first transposon end, leading to IHF ejection and unfolding of the complex. The ejection of IHF promotes cleavage of the second end, which is followed by restoration of the high affinity state which in turn regulates target interactions.

Architecture of recombination intermediates visualized by in-gel FRET of lambda integrase-Holliday junction-arm DNA complexes

Lambda integrase (Int) mediates recombination between attachment sites on phage and Escherichia coli DNA. Int is assisted by accessory protein-induced DNA loops in bridging pairs of distinct "arm-type" and "core-type" DNA sites to form synapsed recombination complexes that subsequently recombine by means of a Holliday junction (HJ) intermediate. An in-gel FRET assay was developed and used to measure 15 distances between six points in two Int-HJ complexes containing arm-DNA oligonucleotides, and 3D maps of these complexes were derived by distance-geometry calculations. The maps reveal unexpected positions for the arm-type DNAs relative to core sites on the HJ and a new Int conformation in the HJ tetramer. The results show how the position of arm DNAs determines the bias of catalytic activities responsible for directional resolution, provide insights into the organization of Int higher-order complexes, and lead to models of the structure of the full HJ recombination intermediates.

Non-equivalent interactions between amino-terminal domains of neighboring lambda integrase protomers direct Holliday junction resolution

The bacteriophage lambda site-specific recombinase (Int), in contrast to other family members such as Cre and Flp, has an amino-terminal domain that binds "arm-type" DNA sequences different and distant from those involved in strand exchange. This defining feature of the heterobivalent recombinases confers a directionality and regulation that is unique among all recombination pathways. We show that the amino-terminal domain is not a simple "accessory" element, as originally thought, but rather is incorporated into the core of the recombination mechanism, where it is well positioned to exert its profound effects. The results reveal an unexpected pattern of intermolecular interactions between the amino-terminal domain of one protomer and the linker region of its neighbor within the tetrameric Int complex and provide insights into those features distinguishing an "active" from an "inactive" pair of Ints during Holliday junction resolution.

The efficiency of mispaired ligations by lambda integrase is extremely sensitive to context

The integrase protein (Int) of phage lambda is a well-studied representative of the tyrosine recombinase family, whose defining features are two sequential pairs of DNA cleavage/ligation reactions that proceed via a 3' phosphotyrosine covalent intermediate to first form and then resolve a Holliday junction recombination intermediate. We devised an assay that takes advantage of DNA hairpin formation at one Int target site to trap Int cleavages at a different target site, and thereby reveal iterative cycles of cleavage and ligation that would otherwise be undetected. Using this assay and others to compare wild-type Int and a mutant (R169D) defective in forming proper dimer/tetramer interfaces, we found that the efficiency of "bottom-strand" DNA cleavage by wild-type Int, but not R169D, is very sensitive to the base-pair at the "top-strand" cleavage site, seven base-pairs away. We show that this is related to the finding that hairpin formation involving ligation of a mispaired base is much faster for R169D than for wild-type Int, but only in the context of a multimeric complex. During resolution of Holliday junction recombination intermediates, wild-type Int, but not R169D, is very sensitive to homology at the sites of ligation. A long-sought insight from these results is that during Holliday junction resolution the tetrameric Int complex remains intact until after ligation of the product helices has been completed. This contrasts with models in which the second pair of DNA cleavages is a trigger for dissolution of the recombination complex.

DNA looping and catalysis; the IHF-folded arm of Tn10 promotes conformational changes and hairpin resolution

DNA loops and bends are common features of DNA processing machines. The bacterial transposon Tn10 has recruited integration host factor (IHF), a site-specific DNA-bending protein, as an architectural component for assembly of the higher-order nucleoprotein complex within which the transposition reaction takes place. Here, we demonstrate additional roles for the IHF loop during the catalytic steps of the reaction. We show that metal ion-dependent unfolding of the IHF-bent transposon arm is communicated to the catalytic center, inducing a substantial conformational change in the DNA. Partial disruption of the IHF loop shows that this step promotes resolution of the hairpin intermediate on one transposon end and initiation of catalysis at the other. Further evidence suggests that the molecular mechanism responsible may be mechanical stress in the IHF loop, related to a change in the relative position of the transposase contacts that anchor the loop on either side.

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.

Activation of site-specific DNA integration in human cells by a single chain integration host factor

The heterodimeric integration host factor (IHF) is a site-specific DNA-binding and DNA-bending protein from Escherichia coli. It plays essential roles in a variety of DNA transactions including recombination, transcription and DNA replication. IHF's ability for concerted binding and bending of DNA is key to its biological function. Here we report the design, characterization and application of a single polypeptide chain IHF, termed scIHF2. In a novel approach for protein engineering, we inserted almost the entire alpha-subunit of IHF into the beta-subunit. DNA binding and DNA bending assays revealed that purified wild-type IHF and scIHF2 behave very similarly. Further, scIHF2 is required for site-specific integrative recombination by phage lambda integrase and for pSC101 replication in a DeltaIHF E.coli host. It also triggers site-specific integrative and excisive recombination in vitro to the same extent as the wild-type protein. We also demonstrate that scIHF2 is stably expressed in HeLa cells, that it is localized primarily in the cell nucleus and that it triggers integrative recombination in mammalian cells by wild-type integrase. Hence, scIHF2 may be used as a novel regulatory cofactor for recombination or other DNA transactions in mammalian cells that require or benefit from sequence-specific high precision DNA bending.

A conformational switch controls the DNA cleavage activity of lambda integrase

The bacteriophage lambda integrase protein (lambda Int) belongs to a family of tyrosine recombinases that catalyze DNA rearrangements. We have determined a crystal structure of lambda Int complexed with a cleaved DNA substrate through a covalent phosphotyrosine bond. In comparison to an earlier unliganded structure, we observe a drastic conformational change in DNA-bound lambda Int that brings Tyr342 into the active site for cleavage of the DNA in cis. A flexible linker connects the central and the catalytic domains, allowing the protein to encircle the DNA. Binding specificity is achieved through direct interactions with the DNA and indirect readout of the flexibility of the att site. The conformational switch that activates lambda Int for DNA cleavage exposes the C-terminal 8 residues for interactions with a neighboring Int molecule. The protein interactions mediated by lambda Int's C-terminal tail offer a mechanism for the allosteric control of cleavage activity in higher order lambda Int complexes.