The λ Red Proteins Promote Efficient Recombination between Diverged Sequences: Implications for Bacteriophage Genome Mosaicism

Genome mosaicism in temperate bacterial viruses (bacteriophages) is so great that it obscures their phylogeny at the genome level. However, the precise molecular processes underlying this mosaicism are unknown. Illegitimate recombination has been proposed, but homeologous recombination could also be at play. To test this, we have measured the efficiency of homeologous recombination between diverged oxa gene pairs inserted into λ. High yields of recombinants between 22% diverged genes have been obtained when the virus Red Gam pathway was active, and 100 fold less when the host Escherichia coli RecABCD pathway was active. The recombination editing proteins, MutS and UvrD, showed only marginal effects on λ recombination. Thus, escape from host editing contributes to the high proficiency of virus recombination. Moreover, our bioinformatics study suggests that homeologous recombination between similar lambdoid viruses has created part of their mosaicism. We therefore propose that the remarkable propensity of the λ-encoded Red and Gam proteins to recombine diverged DNA is effectively contributing to mosaicism, and more generally, that a correlation may exist between virus genome mosaicism and the presence of Red/Gam-like systems.     

The λ Red Proteins Promote Efficient Recombination between Diverged Sequences: Implications for Bacteriophage Genome Mosaicism

Genome mosaicism in temperate bacterial viruses (bacteriophages) is so great that it obscures their phylogeny at the genome level. However, the precise molecular processes underlying this mosaicism are unknown. Illegitimate recombination has been proposed, but homeologous recombination could also be at play. To test this, we have measured the efficiency of homeologous recombination between diverged oxa gene pairs inserted into λ. High yields of recombinants between 22% diverged genes have been obtained when the virus Red Gam pathway was active, and 100 fold less when the host Escherichia coli RecABCD pathway was active. The recombination editing proteins, MutS and UvrD, showed only marginal effects on λ recombination. Thus, escape from host editing contributes to the high proficiency of virus recombination. Moreover, our bioinformatics study suggests that homeologous recombination between similar lambdoid viruses has created part of their mosaicism. We therefore propose that the remarkable propensity of the λ-encoded Red and Gam proteins to recombine diverged DNA is effectively contributing to mosaicism, and more generally, that a correlation may exist between virus genome mosaicism and the presence of Red/Gam-like systems.     

Activation of recF-dependent recombination in Escherichia coli by bacteriophage lambda- and P22-encoded functions.

Escherichia coli strains bearing wild-type and mutant alleles of various recombination genes, as well as plasmids that express recombination-related genes of bacteriophages lambda and P22, were tested for their proficiency as recipients in Hfr-mediated conjugation. It was found that the homologous recombination systems of both phages could promote recombination in a recB recC mutant host. In addition, the Abc function of P22, but not the Gam function of lambda, was found to inhibit recombination in a wild-type host; however, both Abc and Gam inhibited recombination in a recF mutant host. These observations are interpreted as indicating that the recombination systems of both phages, as well as the RecBCD-modulating functions Abc and Gam, all activate the RecF recombination pathway of E. coli.     

Scarless and sequential gene modification in <Emphasis Type="Italic">Pseudomonas</Emphasis> using PCR product flanked by short homology regions

Background  

The lambda Red recombination system has been used to inactivate chromosomal genes in various bacteria and fungi. The procedure consists of electroporating a polymerase chain reaction (PCR) fragment containing antibiotic cassette flanked by homology regions to the target locus into a strain that can express the lambda Red proteins (Gam, Bet, Exo).     

Improvement of recombination efficiency by mutation of red proteins

The Red recombination system of the bacteriophage lambda is a useful tool for engineering bacterial artificial chromosomes (BACs) because desired mutations can be made in any part of the DNA, even in large DNAs, independent of the presence of appropriate restriction enzyme sites. Many have tried changing the inducible promoter or decreasing the copy number of Red genes in Escherichia coli to improve the recombination efficiency. Here we describe a novel method for increasing the recombination efficiency of the Red system by mutating the Red proteins. Mutant Red alpha and Red beta independently increased recombination efficiency. We found that the 18-bp 5' untranslated region (UTR) upstream from the ATG initiation codon of Gam protein is important for recombination efficiency. Although low rates of recombination have been observed in systems using high copy number plasmids, when we performed Red recombination using a high copy number plasmid containing 5'-UTR, mutant Red alpha, and mutant Red beta, recombination increased approximately 145-fold. The novel possibility that mutant Red alpha and Red beta can increase recombination efficiency indicates a new direction for improving recombination efficiency of expression systems that use low or high copy number plasmids or a defective prophage integrated into the E. coli chromosome.     

Mismatch repair and homeologous recombination

DNA mismatch repair influences the outcome of recombination events between diverging DNA sequences. Here we discuss how mismatch repair proteins are active in different homologous recombination subpathways and specific reaction steps, resulting in differential modulation of these recombination events, with a focus on the mechanism of heteroduplex rejection during the inhibition of recombination between slightly diverged (homeologous) DNA sequences.     

Use of a bioluminescence gene reporter for the investigation of red-dependent and gam-dependent plasmid recombination in Escherichia coli K12

A plasmid recombination assay, which utilized mutated Vibrio fischeri luciferase genes, cloned in Escherichia coli plasmids was developed. Expression of the recombination product, a functional luxA gene, was assayed by measuring light intensity. This system was used to investigate the effect of E. coli gene functions on lambda Red- and Gam-dependent plasmid recombination. The genetic and physiological requirements for Red- and Gam-dependent plasmid recombination are similar to the conditions which allow synthesis of plasmid linear multimers. Both recombination and linear multimer synthesis are mediated by Red activity in recBrecC and in sbcB mutants and by Gam activity in sbcB and sbcA mutants, but neither recombination nor linear multimer synthesis is mediated by Red or Gam functions in RecBCD+ExoI+ cells. When mediated by Red in sbcB mutants, both recombination and linear multimer synthesis are RecA-independent, and when mediated by Gam, in the same genetic background, both are RecA-dependent. A role for replication in Red- and Gam-mediated plasmid recombination is suggested by the dependence of the recombination activity on DnaB. A model which hypothesizes mutual dependence of linear plasmid multimer synthesis and plasmid recombination by the RecE, RecF and Red pathways is presented. We propose that ends that are produced during this type of replication are recombinogenic in all three pathways and that new rounds of replication are primed by a recombination-dependent invasion of duplex DNA by 3' single strand ends.     

Control of Large Chromosomal Duplications in Escherichia Coli by the Mismatch Repair System

Excessive recombination between repeated, interspersed, and diverged DNA sequences is a potential source of genomic instability. We have investigated the possibility that a mechanism exists to suppress genetic exchange between these quasi-homologous (homeologous) sequences. We examined the role of the general mismatch repair system of Escherichia coli because previous work has shown that the mismatch repair pathway functions as a barrier to interspecies recombination between E. coli and Salmonella typhimurium. The formation of large duplications by homeologous recombination in E. coli was increased some tenfold by mutations in the mutL and mutS genes that encode the mismatch recognition proteins. These findings indicate that the mismatch recognition proteins act to prevent excessive intrachromosomal exchanges. We conclude that mismatch repair proteins serve as general controllers of the fidelity of genetic inheritance, acting to suppress chromosomal rearrangements as well as point mutations.     

The role of DNA recombination in herpes simplex virus DNA replication

In many organisms the processes of DNA replication and recombination are closely linked. For instance, in bacterial and eukaryotic systems, replication forks can become stalled or damaged, in many cases leading to the formation of double stranded breaks. Replication restart is an essential mechanism in which the recombination and repair machinery can be used to continue replication after such a catastrophic event. DNA viruses of bacteria such as lambda and T4 also rely heavily on DNA recombination to replicate their genomes and both viruses encode specialized gene products which are required for recombination-dependent replication. In this review, we examine the linkage between replication and recombination in the eukaryotic pathogen, Herpes Simplex Virus Type 1 (HSV-1). The evidence that recombination plays an intrinsic role in HSV-1 DNA replication and the infection process will be reviewed. We have recently demonstrated that HSV-1 encodes two proteins which may be analogous to the lambda phage recombination system, Red(alpha) and beta. The HSV-1 alkaline nuclease, a 5' to 3' exonuclease, and ICP8, a single stranded DNA binding protein, can carry out strand annealing reactions similar to those carried out by the lambda Red system. In addition, evidence suggesting that host recombination proteins may also be important for HSV-1 replication will be reviewed. In summary, it is likely that HSV-1 infection will require both viral and cellular proteins which participate in various pathways of recombination and that recombination-dependent replication is essential for the efficient replication of viral genomes.     

Lambda Gam protein inhibits the helicase and chi-stimulated recombination activities of Escherichia coli RecBCD enzyme.

The lambda Gam protein was isolated from cells containing a Gam-producing plasmid. The purified Gam protein was found to bind to RecBCD without displacing any of its subunits. Gam was shown to inhibit all known enzymatic activities of RecBCD: ATP-dependent single- and double-stranded DNA exonucleases, ATP-independent single-stranded endonuclease, and the ATP-dependent helicase. When produced in vivo, Gam inhibited chi-activated recombination in lambda red gam crosses but had little effect on the host's ability to act as a recipient in conjugational recombination. These experiments suggest that RecBCD possesses an additional "unknown" activity that is resistant to or induced by Gam. Additionally, the expression of Gam in recD mutants sensitizes the host to UV irradiation, indicating that Gam alters one or more of the in vivo activities of RecBC(D-).     

The genetic dependence of RecBCD-Gam mediated double strand end repair in Escherichia coli

The repair of double strand breaks after gamma-irradiation in wild-type Escherichia coli lysogenic for lambda cI857 red3 is more efficient when lambda Gam protein is present. This phenomenon, called gam dependent radioresistance, requires the interaction of RecBCD enzyme and Gam protein. We compared cell survival after gamma-irradiation in wild-type and mutant lysogens with and without induction of Gam by transient heat treatment of the cells (6 min, 42 degrees C). The main conclusions are: (1) the RecBCD-Gam pathway of recombination repair is similar but not equivalent to RecBCD, a pathway operating in recD mutants; (2) the RecBCD-Gam pathway is dependent on recJ, recQ and recN gene products and it is proposed that the RecBCD-Gam complex has ability to load RecA protein onto single strand DNA.     

Modulation of Escherichia coli RecBCD activity by the bacteriophage lambda Gam and P22 Abc functions.

Plasmids that express the bacteriophage lambda gam gene or the P22 abc2 gene (with and without abc1) at controllable levels were placed in Escherichia coli and tested for effects on the activity of RecBCD. Like Gam, Abc2 inhibited the ATP-dependent exonuclease activity of RecBCD, apparently not by binding to DNA. However, Abc2-mediated inhibition was partial, while Gam-mediated inhibition was complete. Both Abc2 and Gam inhibited host system-mediated homologous recombination in a Chi-containing interval in the chromosome of a hybrid lambda phage; Abc2 inhibited it more strongly than Gam. Gam but not Abc2 spared a phage T4 gene 2 mutant from restriction by RecBCD; Abc2 exhibited weak sparing activity in combination with Abc1 and substantial activity in combination with both Abc1 and P22 homologous recombination function Erf. Either Gam or the combination of the lambda recombination functions Exo and Bet was sufficient to induce a mode of plasmid replication that produced linear multimers. The combination of Abc2, Abc1, and Erf also exhibited this activity. However, Erf was inactive, both by itself and in combination with Abc1; Abc2 had weak activity. These results indicate that Gam and Abc2 modulate the activity of RecBCD in significantly different ways. In comparison with lambda Gam, P22 Abc2 has a weak effect on RecBCD nuclease activity but a strong effect on its recombination-promoting activity.     

Mechanism and control of interspecies recombination in Escherichia coli. I. Mismatch repair,methylation, recombination and replication functions.

A genetic analysis of interspecies recombination in Escherichia coli between the linear Hfr DNA from Salmonella typhimurium and the circular recipient chromosome reveals some fundamental aspects of recombination between related DNA sequences. The MutS and MutL mismatch binding proteins edit (prevent) homeologous recombination between these 16% diverged genomes by at least two distinct mechanisms. One is MutH independent and presumably acts by aborting the initiated recombination through the UvrD helicase activity. The RecBCD nuclease might contribute to this editing step, presumably by preventing reiterated initiations of recombination at a given locus. The other editing mechanism is MutH dependent, requires unmethylated GATC sequences, and probably corresponds to an incomplete long-patch mismatch repair process that does not depend on UvrD helicase activity. Insignificant effects of the Dam methylation of parental DNAs suggest that unmethylated GATC sequences involved in the MutH-dependent editing are newly synthesized in the course of recombination. This hypothetical, recombination-associated DNA synthesis involves PriA and RecF functions, which, therefore, determine the extent of MutH effect on interspecies recombination. Sequence divergence of recombining DNAs appears to limit the frequency, length, and stability of early heteroduplex intermediates, which can be stabilized, and the recombinants mature via the initiation of DNA replication.     

Bacteriophage P22 Abc2 protein binds to RecC increases the 5' strand nicking activity of RecBCD and together with lambda bet, promotes Chi-independent recombination

Bacteriophage P22 Abc2 protein binds to the RecBCD enzyme from Escherichia coli to promote phage growth and recombination. Overproduction of the RecC subunit in vivo, but not RecB or RecD, interfered with Abc2-induced UV sensitization, revealing that RecC is the target for Abc2 in vivo. UV-induced ATP crosslinking experiments revealed that Abc2 protein does not interfere with the binding of ATP to either the RecB or RecD subunits in the absence of DNA, though it partially inhibits RecBCD ATPase activity. Productive growth of phage P22 in wild-type Salmonella typhimurium correlates with the presence of Abc2, but is independent of the absolute level of ATP-dependent nuclease activity, suggesting a qualitative change in the nature of Abc2-modified RecBCD nuclease activity relative to the native enzyme. In lambda phage crosses, Abc2-modified RecBCD could substitute for lambda exonuclease in Red-promoted recombination; lambda Gam could not. In exonuclease assays designed to examine the polarity of digestion, Abc2 protein qualitatively changes the nature of RecBCD double-stranded DNA exonuclease by increasing the rate of digestion of the 5' strand. In this respect, Abc2-modified RecBCD resembles a RecBCD molecule that has encountered the recombination hotspot Chi. However, unlike Chi-modified RecBCD, Abc2-modified RecBCD still possesses 3' exonuclease activity. These results are discussed in terms of a model in which Abc2 converts the RecBCD exonuclease for use in the P22 phage recombination pathway. This mechanism of P22-mediated recombination distinguishes it from phage lambda recombination, in which the phage recombination system (Red) and its anti-RecBCD function (Gam) work independently.     

Role of mismatch repair in the fidelity of RAD51- and RAD59-dependent recombination in Saccharomyces cerevisiae

To prevent genome instability, recombination between sequences that contain mismatches (homeologous recombination) is suppressed by the mismatch repair (MMR) pathway. To understand the interactions necessary for this regulation, the genetic requirements for the inhibition of homeologous recombination were examined using mutants in the RAD52 epistasis group of Saccharomyces cerevisiae. The use of a chromosomal inverted-repeat recombination assay to measure spontaneous recombination between 91 and 100% identical sequences demonstrated differences in the fidelity of recombination in pathways defined by their dependence on RAD51 and RAD59. In addition, the regulation of homeologous recombination in rad51 and rad59 mutants displayed distinct patterns of inhibition by different members of the MMR pathway. Whereas the requirements for the MutS homolog, MSH2, and the MutL homolog, MLH1, in the suppression of homeologous recombination were similar in rad51 strains, the loss of MSH2 caused a greater loss in homeologous recombination suppression than did the loss of MLH1 in a rad59 strain. The nonequivalence of the regulatory patterns in the wild-type and mutant strains suggests an overlap between the roles of the RAD51 and RAD59 gene products in potential cooperative recombination mechanisms used in wild-type cells.     

The lambda Gam protein inhibits RecBCD binding to dsDNA ends

Inactivation of the Escherichia coli RecBCD enzyme by the lambda Gam protein is an essential step that accompanies the lambda Red proteins for gene replacement using recombineering technology. It has been shown that Gam inhibits all the activities of RecBCD to the same extent. Nonetheless, some in vivo properties of recBCD mutants cannot be mimicked effectively by the expression of gam in vivo. An examination of the mechanism of Gam's inhibition of RecBCD was performed, and it was found that Gam inhibits the binding of RecBCD to double-stranded DNA ends, even if RecBCD is bound to DNA before its interaction with Gam. When ATP is added to the reaction to induce helicase activity, most of the reaction is inhibited by Gam, but residual amounts of unwinding are detected, despite a 40-fold excess of Gam/RecBCD. The same inhibitory effect of Gam was seen on RecBCD that had been modified by the P22 anti-RecBCD protein Abc2, though the inhibitory effect was diminished due to the tighter binding of Abc2-modified RecBCD to double-stranded DNA ends. These data suggest that cells containing Gam-expressing plasmids retain a small amount of uninhibited enzyme. Given the suspected instability of Gam in vivo, care must be taken when interpreting results from experiments containing Gam-inhibited RecBCD species. A revised model is proposed for Gam-induced radioresistance of E. coli to ionizing radiation.     

Modulation of EcoKI restriction in vivo: role of the lambda Gam protein and plasmid metabolism.

Two novel types of alleviation of DNA restriction by the EcoKI restriction endonuclease are described. The first type depends on the presence of the gam gene product (Gam protein) of bacteriophage lambda. The efficiency of plating of unmodified phage lambda is greatly increased when the restricting Escherichia coli K-12 host carries a gam+ plasmid. The effect is particularly striking in wild-type strains and, to a lesser extent, in the presence of sbcC and recA mutations. In all cases, Gam-dependent alleviation of restriction requires active recBCD genes of the host and recombination (red) genes of the infecting phage. The enhanced capacity of Gam-expressing cells to repair DNA strand breaks might account for this phenomenon. The second type is caused by the presence of a plasmid in a restricting host lacking RecBCD enzyme. Commonly used plasmids such as the cloning vector pACYC184 can produce such an effect in strains carrying recB single mutations or in recBC sbcBC strains. Plasmid-mediated restriction alleviation in recBC sbcBC strains is independent of the host RecF, RecJ, and RecA proteins and phage recombination functions. The presence of plasmids can also relieve restriction in recD strains. This effect depends, however, on the RecA function in the host. The molecular mechanism of the plasmid-mediated restriction alleviation remains unclear.     

Bacteriophage SPP1 Chu is an alkaline exonuclease in the SynExo family of viral two-component recombinases

Many DNA viruses concatemerize their genomes as a prerequisite to packaging into capsids. Concatemerization arises from either replication or homologous recombination. Replication is already the target of many antiviral drugs, and viral recombinases are an attractive target for drug design, particularly for combination therapy with replication inhibitors, due to their important supporting role in viral growth. To dissect the molecular mechanisms of viral recombination, we and others previously identified a family of viral nucleases that comprise one component of a conserved, two-component viral recombination system. The nuclease component is related to the exonuclease of phage lambda and is common to viruses with linear double-stranded DNA genomes. To test the idea that these viruses have a common strategy for recombination and genome concatemerization, we isolated the previously uncharacterized 34.1 gene from Bacillus subtilis phage SPP1, expressed it in Escherichia coli, purified the protein, and determined its enzymatic properties. Like lambda exonuclease, Chu (the product of 34.1) forms an oligomer, is a processive alkaline exonuclease that digests linear double-stranded DNA in a Mg(2+)-dependent reaction, and shows a preference for 5'-phosphorylated DNA ends. A model for viral recombination, based on the phage lambda Red recombination system, is proposed.     

Molecular evolution and phylogeny of the human AIDS viruses LAV,HTLV-III,and ARV

Summary A phylogenetic tree for the human lymphadenopathy-associated virus (LAV), the human T-cell lymphotrophic virus type III (HTLV-III), and the acquired immune deficiency syndrome (AIDS)-associated retrovirus (ARV) has been constructed from comparisons of the amino acid sequences of their gag proteins. A method is proposed for estimating the divergence times among these AIDS viruses and the rates of nucleotide substitution for their RNA genomes. The analysis indicates that the LAV and HTLV-III strains diverged from one another after 1977 and that their common ancestor diverged from the ARV virus no more than 10 years earlier. Hence, the evolutionary diversity among strains of the AIDS viruses apparently has been generated within the last 20 years. It is estimated that the genome of the AIDS virus has a nucleotide substitution rate on the order of 10^–3 per site per year, with the rate in the second half of the genome being double that in the first half.     

Recombination every day: abundant recombination in a virus during a single multi-cellular host infection

Viral recombination can dramatically impact evolution and epidemiology. In viruses, the recombination rate depends on the frequency of genetic exchange between different viral genomes within an infected host cell and on the frequency at which such co-infections occur. While the recombination rate has been recently evaluated in experimentally co-infected cell cultures for several viruses, direct quantification at the most biologically significant level, that of a host infection, is still lacking. This study fills this gap using the cauliflower mosaic virus as a model. We distributed four neutral markers along the viral genome, and co-inoculated host plants with marker-containing and wild-type viruses. The frequency of recombinant genomes was evaluated 21 d post-inoculation. On average, over 50% of viral genomes recovered after a single host infection were recombinants, clearly indicating that recombination is very frequent in this virus. Estimates of the recombination rate show that all regions of the genome are equally affected by this process. Assuming that ten viral replication cycles occurred during our experiment—based on data on the timing of coat protein detection—the per base and replication cycle recombination rate was on the order of 2 × 10⁻⁵ to 4 × 10⁻⁵. This first determination of a virus recombination rate during a single multi-cellular host infection indicates that recombination is very frequent in the everyday life of this virus.