Fate of cloned bacteriophage T4 DNA after phage T4 infection of clone-bearing cells

Homologous Recombination-Experimental Systems, Analysis, and Significance

Homologous recombination is the most complex of all recombination events that shape genomes and produce material for evolution. Homologous recombination events are exchanges between DNA molecules in the lengthy regions of shared identity, catalyzed by a group of dedicated enzymes. There is a variety of experimental systems in Escherichia coli and Salmonella to detect homologous recombination events of several different kinds. Genetic analysis of homologous recombination reveals three separate phases of this process: pre-synapsis (the early phase), synapsis (homologous strand exchange), and post-synapsis (the late phase). In E. coli, there are at least two independent pathway of the early phase and at least two independent pathways of the late phase. All this complexity is incongruent with the originally ascribed role of homologous recombination as accelerator of genome evolution: there is simply not enough duplication and repetition in enterobacterial genomes for homologous recombination to have a detectable evolutionary role and therefore not enough selection to maintain such a complexity. At the same time, the mechanisms of homologous recombination are uniquely suited for repair of complex DNA lesions called chromosomal lesions. In fact, the two major classes of chromosomal lesions are recognized and processed by the two individual pathways at the early phase of homologous recombination. It follows, therefore, that homologous recombination events are occasional reflections of the continual recombinational repair, made possible in cases of natural or artificial genome redundancy.

Initiation of bacteriophage T4 DNA replication and replication fork dynamics: a review in the Virology Journal series on bacteriophage T4 and its relatives

Bacteriophage T4 initiates DNA replication from specialized structures that form in its genome. Immediately after infection, RNA-DNA hybrids (R-loops) occur on (at least some) replication origins, with the annealed RNA serving as a primer for leading-strand synthesis in one direction. As the infection progresses, replication initiation becomes dependent on recombination proteins in a process called recombination-dependent replication (RDR). RDR occurs when the replication machinery is assembled onto D-loop recombination intermediates, and in this case, the invading 3' DNA end is used as a primer for leading strand synthesis. Over the last 15 years, these two modes of T4 DNA replication initiation have been studied in vivo using a variety of approaches, including replication of plasmids with segments of the T4 genome, analysis of replication intermediates by two-dimensional gel electrophoresis, and genomic approaches that measure DNA copy number as the infection progresses. In addition, biochemical approaches have reconstituted replication from origin R-loop structures and have clarified some detailed roles of both replication and recombination proteins in the process of RDR and related pathways. We will also discuss the parallels between T4 DNA replication modes and similar events in cellular and eukaryotic organelle DNA replication, and close with some current questions of interest concerning the mechanisms of replication, recombination and repair in phage T4.

Repair of topoisomerase-mediated DNA damage in bacteriophage T4

Type II topoisomerase inhibitors are used to treat both tumors and bacterial infections. These inhibitors stabilize covalent DNA-topoisomerase cleavage complexes that ultimately cause lethal DNA damage. A functional recombinational repair apparatus decreases sensitivity to these drugs, suggesting that topoisomerase-mediated DNA damage is amenable to such repair. Using a bacteriophage T4 model system, we have developed a novel in vivo plasmid-based assay that allows physical analysis of the repair products from one particular topoisomerase cleavage site. We show that the antitumor agent 4'-(9-acridinylamino)methanesulphon-m-anisidide (m-AMSA) stabilizes the T4 type II topoisomerase at the strong topoisomerase cleavage site on the plasmid, thereby stimulating recombinational repair. The resulting m-AMSA-dependent repair products do not form in the absence of functional topoisomerase and appear at lower drug concentrations with a drug-hypersensitive topoisomerase mutant. The appearance of repair products requires that the plasmid contain a T4 origin of replication. Finally, genetic analyses demonstrate that repair product formation is absolutely dependent on genes 32 and 46, largely dependent on genes uvsX and uvsY, and only partly dependent on gene 49. Very similar genetic requirements are observed for repair of endonuclease-generated double-strand breaks, suggesting mechanistic similarity between the two repair pathways.

Two types of recombination hotspots in bacteriophage T4: one requires DNA damage and a replication origin and the other does not

Recombination hotspots have previously been discovered in bacteriophage T4 by two different approaches, marker rescue recombination from heavily damaged phage genomes and recombination during co-infection by two undamaged phage genomes. The phage replication origin ori(34) is located in a region that has a hotspot in both assays. To determine the relationship between the origin and the two kinds of hotspots, we generated phage carrying point mutations that should inactivate ori(34) but not affect the gene 34 reading frame (within which ori(34) is located). The mutations eliminated the function of the origin, as judged by both autonomous replication of plasmids during T4 infection and two-dimensional gel analysis of phage genomic replication intermediates. As expected from past studies, the ori(34) mutations also eliminated the hotspot for marker rescue recombination from UV-irradiated genomes. However, the origin mutations had no effect on the recombination hotspot that is observed with co-infecting undamaged phage genomes, demonstrating that some DNA sequence other than the origin is responsible for inflated recombination between undamaged genomes. The hotspots for marker rescue recombination may result from a replication fork restart process that acts upon origin-initiated replication forks that become blocked at nearby DNA damage. The two-dimensional gel analysis also revealed phage T4 replication intermediates not previously detected by this method, including origin theta forms.

Integration of plasmids into the bacteriophage T4 genome

We have analyzed the integration of plasmids into the bacteriophage T4 genome via homologous recombination. As judged by genetic selection for a plasmid-borne marker, a mutation in phage gene uvsX or uvsY essentially blocked the integration of a plasmid with homology to the T4 genome but no phage replication origin (non-origin plasmid). The strict requirement for these two proteins suggests that plasmid integration can proceed via a strand-invasion reaction similar to that catalyzed in vitro by the T4-encoded strand-exchange protein (UvsX) in concert with UvsY and gp32. In contrast to the results with the non-origin plasmid, a mutation in uvsX or uvsY reduced the integration of a T4 replication origin-containing plasmid by only 3-10-fold. These results suggest that the origin-containing plasmid integrates by both the UvsXY-dependent pathway used by the non-origin plasmid and by a UvsXY-independent pathway. The origin-containing plasmid integrated into the phage genome during a uvsX- or uvsY-mutant infection of a recA-mutant host, and therefore origin-dependent integration can occur in the absence of both phage- and host-encoded strand-exchange proteins (UvsX and RecA, respectively).

Transactivation of a plasmid-borne bacteriophage T4 late gene

We examined how a plasmid-borne T4 late gene is activated by infecting T4 phage (transactivation). A gene fusion system was developed where expression of a late gene promoter fused to the lacZ gene may easily be followed by measuring beta-galactosidase activity. Considerable transactivation can occur, provided that the infecting phage contains a mutation which abolishes the denB-encoded endonuclease, and that the gene 46-encoded exonuclease is functional. The level of transactivation was correlated with the formation of high molecular weight DNA composed of tandem repeats of plasmid DNA. The formation of these molecules and subsequent transactivation depended on DNA replication and homology between phage and plasmid DNAs. Also the capacity of bacteriophage T4, grown on cells containing a plasmid-borne T4 gene, to transduce the plasmid provided indirect evidence of the formation of these tandem-repeat molecules. A good correlation was established between the ability to transduce and the presence of sequence homology between the phage and the plasmid. However, the requirement for phage/plasmid homology is no longer prerequisite if transcription from the plasmid is permitted by introducing an alc mutation into the infecting phage, presumably because this allows DNA replication to start through RNA priming.

Deletion of the essential gene 24 from the bacteriophage T4 genome

We deleted the essential gene 24 from the genome of bacteriophage T4. The delta 24 phage is a conditional lethal mutant that can grow only when the host strain supplies the product of gene 24 in trans, or when the phage acquires a functional gene 24 by some type of recombination event. Thus, gene 24 can be used as a selectable marker, for example permitting transposition into the T4 genome and analyses of plasmid-phage recombination [Woodworth and Kreuzer, Mol. Microbiol. 6 (1992) 1289-1296; H.W.E. and K.N.K., manuscript submitted]. We also found that the promoter region of gene 24 allows a low level of autonomous plasmid replication in T4-infected cells, raising the possibility of a previously unrecognized mode of T4 replication initiation.

Recombination-dependent concatemeric plasmid replication

The replication of covalently closed circular supercoiled (form I) DNA in prokaryotes is generally controlled at the initiation level by a rate-limiting effector. Once initiated, replication proceeds via one of two possible modes (theta or sigma replication) which do not rely on functions involved in DNA repair and general recombination. Recently, a novel plasmid replication mode, leading to the accumulation of linear multigenome-length plasmid concatemers in both gram-positive and gram-negative bacteria, has been described. Unlike form I DNA replication, an intermediate recombination step is most probably involved in the initiation of concatemeric plasmid DNA replication. On the basis of structural and functional studies, we infer that recombination-dependent plasmid replication shares important features with phage late replication modes and, in several aspects, parallels the synthesis of plasmid concatemers in phage-infected cells. The characterization of the concatemeric plasmid replication mode has allowed new insights into the mechanisms of DNA replication and recombination in prokaryotes.

Rifampin-resistant replication of pBR322 derivatives in Escherichia coli cells induced for the SOS response

Replication of plasmid pBR322 in Escherichia coli cells normally requires RNA synthesis and thus is sensitive to rifampin, an inhibitor of RNA polymerase. In cells induced for the SOS response, however, derivatives of pBR322 were found to replicate in the presence of rifampin. This rifampin-resistant replication of pBR322 requires the insertion of certain sequences of DNA. The replication depends on recF+ and DNA polymerase I.

The generation of concatemeric plasmid DNA in Bacillus subtilis as a consequence of bacteriophage SPP1 infection

Bacteriophage SPP1 infection of Bacillus subtilis cells bearing plasmids induces the synthesis of multigenome-length plasmid molecules. Two independent pathways can account for this synthesis. In one of those, homology to the phage genome is required, whereas in the other such homology is not a prerequisite. In wild type cells both modes overlap. In dnaB(Ts), at non permissive temperature, or in recE polA strains the main concatemeric plasmid replication mode is the homology-dependent plasmid (hdp) mode. The rate of recombination-dependent concatemeric plasmid DNA synthesis is a consequence of a phage-plasmid interaction which leads to chimeric phage::plasmid DNA. The second mode, which is an homology-independent plasmid (hip) mode seems to be triggered upon the synthesis of a phage encoded product(s) (e.g. inactivation of the exonuclease V enzyme).

Bacteriophage T4 late transcription from plasmid templates is enhanced by negative supercoiling

Concurrent viral replication is normally required to activate bacteriophage T4 late promoters; replication is thought to provide a template structure which is competent for late transcription. Transcription from plasmid-borne T4 late promoters, however, is independent of replication in vivo and in vitro. In this work, we have shown that, when the late gene 23 promoter is located on a plasmid, its utilization in vivo depends upon the ability of host DNA gyrase to maintain some degree of negative superhelicity. This suggests that an alternative pathway exists for activation of late promoters: DNA which is under sufficient negative torsional stress is already competent for late transcription. We also describe a method for isolating ternary complexes of plasmid DNA, RNA polymerase, and nascent RNA which have initiated transcription in vivo. The topoisomer distribution of such ternary complexes prepared from T4-infected cells showed that, late in infection, transcriptional activity resides primarily in the subset of the plasmid population with the most negatively supercoiled topoisomers. However, the overall transcriptional pattern in these ternary complexes indicated that both vector and T4 sequences are actively transcribed. Much of this transcriptional activity could be independent of gp55, the T4-specific RNA polymerase-binding protein that confers late promoter recognition.

Topoisomerization of plasmid DNA in Escherichia coli infected with bacteriophage T4

The degradation of host DNA, and the block to transcription of cytosine-containing DNA, which are a part of the normal course of infection by bacteriophage T4, can be eliminated in an appropriate T4 genetic background (designated as our reference type, or r.t.), so that T4 late promoters carried on plasmid DNA can function. The changes of topoisomer distribution that ensue when phage T4 r.t. infect Escherichia coli carrying a plasmid containing a T4 late promoter were analyzed. The linking number of the covalently closed circular plasmid DNA increased (implying relaxation) at the same time as the distribution of topoisomers became much broader. The relaxation of plasmid DNA was primarily, but not exclusively, due to T4 DNA topoisomerase II. The bacterial DNA topoisomerase II (gyrase) continued to function after phage infection to maintain some degree of superhelicity in plasmid DNA. When the DNA gyrase was inhibited by coumermycin or oxolinic acid, the topoisomer distribution became distinctly bimodal, part of the DNA remaining highly negatively supercoiled. It is argued that the observed post-infection topological changes involve relaxation of torsional stress and changes of binding by proteins that topologically constrain the plasmid DNA.

Characterization of a defective phage system for the analysis of bacteriophage T4 DNA replication origins

We have developed a defective phage system for the isolation and analysis of phage T4 replication origins based on the T4-mediated transduction of plasmid pBR322. During the initial infection of a plasmid-containing cell, recombinant plasmids with T4 DNA inserts are converted into fully modified linear DNA concatamers that are packaged into T4 phage particles, to create defective phage (transducing particles). In order to select T4 replication origins from genomic libraries of T4 sequences cloned into the plasmid pBR322, we searched for recombinant plasmids that transduce with an unusually high efficiency, reasoning that this should select for T4 sequences that function as origins on plasmid DNA after phage infection. We also selected for defective phage that can propagate efficiently with the aid of a coinfecting helper phage during subsequent rounds of phage infection, which should select for T4 sequences that can function as origins on the linear DNA present in the defective phage. Several T4 inserts were isolated repeatedly in one or both of these selective procedures, and these were mapped to particular locations on the T4 genome. When plasmids were selected in this way from genomic libraries constructed using different restriction nucleases, they contained overlapping segments of the T4 genome, indicating that the same T4 sequences were selected. The inserts in two of the selected plasmids permit a very high frequency of transduction from circular plasmids; these have been shown to contain a special type of T4 replication origin.

Expression of a cloned denV gene of bacteriophage T4 in Escherichia coli

A 713-base-pair Hae III fragment from bacteriophage T4 encompassing the denV gene with its preceding promoter has been cloned in a pBR322-derived positive-selection vector and introduced into a variety of DNA repair-deficient uvr and rec and uvr,rec Escherichia coli strains. The denV gene was found to be expressed, probably from its own promoter, causing pyrimidine dimer incision-deficient uvrA, uvrB, uvrC strains to be rescued by the denV gene. A uvrD (DNA helicase II) strain was also complemented, but to a lesser extent. A wild-type strain did not seem to be affected at the UV doses tested. Surprisingly, all recA, recB, and recC strains tested also showed an increased UV resistance, perhaps by reinforcement of the intact uvr system in these strains. Complementation of denV- T4 strains and host-cell reactivation of lambda phage was also observed in denV+ E. coli strains. Equilibrium sedimentation showed that DNA repair synthesis occurred in a UV-irradiated uvrA E. coli strain carrying the cloned denV gene. Southern blotting confirmed our earlier results [Valerie, K., Henderson, E. E. & de Riel, J. K. (1984) Nucleic Acids Res. 12, 8085-8096] that the denV gene is located at 64 kilobases on the T4 map. Phage T2 (denV-) did not hybridize to a denV-specific probe.

Plasmid transduction by Bacillus subtilis bacteriophage SPP1: effects of DNA homology between plasmid and bacteriophage

Any SPP1 DNA restriction fragment cloned into Bacillus subtilis plasmid pC194 or pUB110 increased the transduction frequency of the plasmid by SPP1 100- to 1,000-fold over the transduction level of the plasmid alone. This increment was observed irrespective of whether a fragment contained the SPP1 packaging origin (pac). Furthermore, an SPP1 derivative into whose genome pC194 DNA had been integrated transduced pC194 DNA with a greatly enhanced frequency. Transduction enhancement mediated by DNA-DNA homology between plasmid and SPP1 was independent of the extent of homology (size range analyzed, 0.5 to 3.9 kilobases) and the recombination proficiency of donor or recipient.

A defective phage system reveals bacteriophage T4 replication origins that coincide with recombination hot spots

Plasmid transduction mediated by bacteriophage T4 has been used to study putative T4 DNA replication origins cloned as inserts in the Escherichia coli plasmid pBR322. Two particular inserts from the T4 genome allow high-frequency plasmid transduction, suggesting that each insert might contain a T4 replication origin. T4 infection of these plasmid-containing cells produces large numbers of defective phage particles that contain long linear concatamers of the plasmid DNA. During a second cycle of infection, these defective phage genomes can be replicated better than normal phage chromosomes present in the same infected cell; consequently, the T4 DNA inserts must be functioning as replication origins. Both of these origins appear to utilize a previously unrecognized mode of T4 replication initiation. Moreover, each origin coincides with a major recombination hot spot in the phage genome, and therefore this mode of replication initiation seems to involve a local stimulation of homologous genetic recombination. From a purely practical standpoint, additional DNA fragments can be cloned in an origin-containing plasmid, allowing isolation of large amounts of any DNA sequence with the glucosylated hydroxymethylcytosine modifications of T4 DNA.

Recombination between bacteriophage T4 and plasmid pBR322 molecules containing cloned T4 DNA

Reciprocal recombination between T4 DNA cloned in plasmid pBR322 and homologous sequences in bacteriophage T4 genomes leads to integration of complete plasmid molecules into phage genomes. Indirect evidence of this integration comes from two kinds of experiments. Packaging of pBR322 DNA into mature phage particles can be detected by a DNA--DNA hybridization assay only when a T4 restriction fragment is cloned in the plasmid. The density of the pBR322 DNA synthesized after phage infection is also consistent with integration of plasmid vector DNA into vegetative phage genomes. Direct evidence of plasmid integration into phage genomes in the region of DNA homology comes from genetic and biochemical analysis of cytosine-containing DNA isolated from mature phage particles. Agarose gel electrophoresis of restriction endonuclease-digested DNA, followed by Southern blot analysis with nick-translated probes, shows that entire plasmid molecules become integrated into phage genomes in the region of T4 DNA homology. In addition, this analysis shows that genomes containing multiple copies of complete plasmid molecules are also formed. Among phage particles containing at least one integrated copy, the average number of integrated plasmid molecules is almost ten. A cloning experiment done with restricted DNA confirms these conclusions and illustrates a method for walking along the T4 genome.