Genetic structure of the replication origin of bacteriophage lambda

Characterization of a newly discovered putative DNA replication initiator from Paenibacillus polymyxa phage phiBP

The bacteriophage phiBP contains a newly discovered putative replisome organizer, a helicase loader, and a beta clamp, which together may serve to replicate its DNA. Bioinformatics analysis of the phiBP replisome organizer sequence showed that it belongs to a recently identified family of putative initiator proteins. We prepared and isolated a wild type-like recombinant protein, gpRO-HC, and a mutant protein gpRO-HCK8A, containing a lysine to alanine substitution at position 8. gpRO-HC had low ATPase activity regardless of the presence of DNA, while the ATPase activity of the mutant was significantly higher. gpRO-HC bound to both single- and double-stranded DNA substrates. Different methods showed that gpRO-HC forms higher oligomers containing about 12 subunits. This work provides the first information about another group of phage initiator proteins, which trigger DNA replication in phages infecting low GC Gram-positive bacteria.

Complementation Studies of Bacteriophage λ O Amber Mutants by Allelic Forms of O Expressed from Plasmid, and O-P Interaction Phenotypes

λ genes O and P are required for replication initiation from the bacteriophage λ origin site, oriλ, located within gene O. Questions have persisted for years about whether O-defects can indeed be complemented in trans. We show the effect of original null mutations in O and the influence of four origin mutations (three are in-frame deletions and one is a point mutation) on complementation. This is the first demonstration that O proteins with internal deletions can complement for O activity, and that expression of the N-terminal portion of gene P can completely prevent O complementation. We show that O-P co-expression can limit the lethal effect of P on cell growth. We explore the influence of the contiguous small RNA OOP on O complementation and P-lethality.

Lambda gpP-DnaB Helicase Sequestration and gpP-RpoB Associated Effects: On Screens for Auxotrophs, Selection for Rif(R), Toxicity, Mutagenicity, Plasmid Curing

The bacteriophage lambda replication initiation protein P exhibits a toxic effect on its Escherichia coli (E. coli) host, likely due to the formation of a dead-end P-DnaB complex, sequestering the replicative DnaB helicase from further activity. Intracellular expression of P triggers SOS-independent cellular filamentation and rapidly cures resident ColE1 plasmids. The toxicity of P is suppressed by alleles of P or dnaB. We asked whether P buildup within a cell can influence E. coli replication fidelity. The influence of P expression from a defective prophage, or when cloned and expressed from a plasmid was examined by screening for auxotrophic mutants, or by selection for rifampicin resistant (Rif(R)) cells acquiring mutations within the rpoB gene encoding the β-subunit of RNA polymerase (RNAP), nine of which proved unique. Using fluctuation assays, we show that the intracellular expression of P evokes a mutator effect. Most of the Rif(R) mutants remained P(S) and localized to the Rif binding pocket in RNAP, but a subset acquired a P(R) phenotype, lost sensitivity to ColE1 plasmid curing, and localized outside of the pocket. One P(R) mutation was identical to rpo*Q148P, which alleviates the UV-sensitivity of ruv strains defective in the migration and resolution of Holliday junctions and destabilizes stalled RNAP elongation complexes. The results suggest that P-DnaB sequestration is mutagenic and supports an earlier observation that P can interact with RNAP.

A CI-independent form of replicative inhibition: turn off of early replication of bacteriophage lambda

Several earlier studies have described an unusual exclusion phenotype exhibited by cells with plasmids carrying a portion of the replication region of phage lambda. Cells exhibiting this inhibition phenotype (IP) prevent the plating of homo-immune and hybrid hetero-immune lambdoid phages. We have attempted to define aspects of IP, and show that it is directed to repλ phages. IP was observed in cells with plasmids containing a λ DNA fragment including oop, encoding a short OOP micro RNA, and part of the lambda origin of replication, oriλ, defined by iteron sequences ITN1-4 and an adjacent high AT-rich sequence. Transcription of the intact oop sequence from its promoter, p(O) is required for IP, as are iterons ITN3-4, but not the high AT-rich portion of oriλ. The results suggest that IP silencing is directed to theta mode replication initiation from an infecting repλ genome, or an induced repλ prophage. Phage mutations suppressing IP, i.e., Sip, map within, or adjacent to cro or in O, or both. Our results for plasmid based IP suggest the hypothesis that there is a natural mechanism for silencing early theta-mode replication initiation, i.e. the buildup of λ genomes with oop(+)oriλ(+) sequence.

Acquired mutations in phage lambda genes O or P that enable constitutive expression of a cryptic lambdaN+cI[Ts]cro- prophage in E. coli cells shifted from 30 degreesC to 42 degreesC, accompanied by loss of immlambda and Rex+ phenotypes and emergence of a non-immune exclusion-state

The majority of bacteria, which carry the N+-cI857[Ts]-cro--O+-P+ fragment of lambda genome, are killed when derepressed by shifting from 30 degreesC to 42 degreesC. Among rare survivors, we observed a proportion of colony-forming units (cfu) that retained the typical immlambda-immunity phenotype when grown at 30 degreesC; however, when shifted from 30 degreesC to 42 degreesC, they lost lambda immunity and acquired a non-immune exclusion-state (Nie phenotype). We also found that the immlambda survivor cfu quickly lost their Rex+ exclusion phenotype (as measured by T4rII plating inhibition) when shifted from 30 degreesC to 42 degreesC, even though they produced CII, which stimulates pE-cI-rexA-rexB transcription. The Nie phenotype was characterized by an inhibition of plating of the homoimmune phage, lambdawt, and the heteroimmune phage, lambdaimm434. However, lambdavir and spontaneous mutants of lambdawt (lambdase mutations localized within oR) escaped the Nie exclusion-state and plated efficiently on lawns of Nie cfu at 42 degreesC. Thus, we examined the scope of the Nie exclusion-state toward lambda mutants blocked for lysogeny, and lambda hybrids substituted for immunity or replication genes. Phage like lambdawt, competent for lysogeny, were severely excluded compared to some mutants of lambda defective for lysogeny. Among this latter type, there was high variance in the Nie exclusion of various cI mutants; some of which were not excluded. The Nie exclusion-state was attributed to the constitutive expression of the defective lambda fragment in the survivor cfu, made possible by the acquired replication defect(s). We characterized, both genetically and physically, the mutations in the defective integrated lambda prophage that permitted growth of the survivor cfu at 42 degreesC. In five of seven survivor cfu, we identified IS2 insertions within lambda genes O and P that can block replication initiation from the lambda fragment. The remaining survivor cfu had multiple base substitutions within the C-terminal end of O and N-terminal half of P, the majority of which were silent. In some of these mutants, either an ochre nonsense mutation or a single-base frameshift deletion inactivated P.

Protein tracking-induced supercoiling of DNA: a tool to regulate DNA transactions in vivo?

An interplay between DNA-dependent biological processes appears to be crucial for cell viability. At the molecular level, this interplay relies heavily on the communication between DNA-bound proteins, which can be facilitated and controlled by the dynamic structure of double-stranded DNA. Hence, DNA structural alterations are recognized as potential tools to transfer biological information over some distance within a genome. Until recently, however, direct evidence for DNA structural information as a mediator between cellular processes was lacking. This changed when the concept of transient waves of DNA supercoiling, induced by proteins tracking along the right-handed DNA double helix, came into the limelight. Indeed, a number of observations now suggest that helix tracking-induced DNA structural information might be exploited to participate in the regulation of a variety of DNA transactions in vivo.

RNA-primed initiation sites of DNA replication in the origin region of bacteriophage lambda genome

Using DNA molecules synthesized in the early stage of lambda phage infection, deoxynucleotides at the transition sites from primer RNA to DNA synthesis have been mapped in the 1.5 kbase area of the lambda phage genome containing the genetically defined replication origin (ori lambda). Sites in the 1-strand (the polarity of the 1-strand is 5' to 3' from the left to the right direction of the lambda phage genetic map) were distributed both inside and outside of the ori lambda, whereas the sites in the r-strand (the strand in the opposite polarity) were mainly distributed more than three hundred nucleotides apart from the ori lambda to the right. A CPuPu sequence was found at -12 to -10 region of transition sites of the r- and the 1-strands in the frequency of 80% and 70%, respectively, and over 60% of the CPuPu sequences were CAG. Properties of the transition sites are discussed in relation to the primer synthesis.

Sequence requirements of Escherichia coli attTn7, a specific site of transposon Tn7 insertion

Transposon Tn7 transposes at high frequency to a specific site, attTn7, in the Escherichia coli chromosome. We devised a quantitative assay for Tn7 transposition in which Tn7-end derivatives containing the cis-acting transposition sequences of Tn7 transpose from a bacteriophage lambda vector upon infection into cells containing the Tn7-encoded transposition proteins. We used this assay to identify a 68-base-pair DNA segment containing the sequences essential for attTn7 target activity. This segment is positioned asymmetrically with respect to the specific point of Tn7 insertion in attTn7 and lacks obvious homology to the sequences at the ends of Tn7 which participate directly in transposition. We also show that some sequences essential for attTn7 target activity are contained within the protein-coding sequence of a bacterial gene.

Specialized nucleoprotein structures at the origin of replication of bacteriophage lambda: localized unwinding of duplex DNA by a six-protein reaction

The O protein of bacteriophage lambda localizes the initiation of DNA replication to a unique site on the lambda genome, ori lambda. By means of electron microscopy, we infer that the binding of O to ori lambda initiates a series of protein addition and transfer reactions that culminate in localized unwinding of the origin DNA, generating a prepriming structure for the initiation of DNA replication. We can define three stages of this prepriming reaction, the first two of which we have characterized previously. First, dimeric O protein binds to multiple DNA binding sites and self-associates to form a nucleoprotein structure, the O-some. Second, lambda P and host DnaB proteins interact with the O-some to generate a larger complex that includes additional DNA from an A + T-rich region adjacent to the O binding sites. Third, the addition of the DnaJ, DnaK, and Ssb proteins and ATP results in an origin-specific unwinding reaction, probably catalyzed by the helicase activity of DnaB. The unwinding reaction is unidirectional, proceeding "rightward" from the origin. The minimal DNA sequence competent for unwinding consists of two O binding sites and the adjacent A + T-rich region to the right of the binding sites. We conclude that the lambda O protein localizes and initiates a six-protein sequential reaction responsible for but preceding the precise initiation of DNA replication. Specialized nucleoprotein structures similar to the O-some may be a general feature of DNA transactions requiring extraordinary precision in localization and control.

Specialized nucleoprotein structures at the origin of replication of bacteriophage lambda: complexes with lambda O protein and with lambda O, lambda P, and Escherichia coli DnaB proteins

The O protein of bacteriophage lambda is required for initiation of DNA replication at the lambda replicative origin designated ori lambda. The binding sites for O protein are four direct repeats, each of which is an inverted repeat. By means of electron microscopy, we have found that phage lambda O protein utilizes these multiple binding sites to form a specific nucleoprotein structure in which the origin DNA is inferred to be folded or wound. The phage lambda O and P proteins and host DnaB protein interact at ori lambda to generate a larger structure than that formed by O protein alone; P and DnaB proteins fail to form any observable complex when O protein is excluded from the reaction mixture. We conclude that the specialized nucleoprotein structure formed by phage lambda O protein and ori lambda provides for localized initiation of DNA replication by serving as the foundation for the assembly of the initial priming structure. Specialized nucleoprotein structures may be a general means to confer exceptional accuracy on DNA transactions requiring extraordinary precision.

Genetic recombination of Xenopus laevis 5 S DNA in bacteria

The behavior in genetic recombination of Xenopus laevis 5 S DNA has been examined, with particular emphasis on the role of 15-base-pair tandem repeats in the A + T-rich spacer. Fragments of 5 S DNA were introduced into Escherichia coli cells as inserts in the recombination vectors, lambda rva and lambda rvb. Intermolecular recombinants were selected in which, because of properties of the phage vectors, the crossover event must have occurred within the 5 S DNA inserts. Inserts from individual recombinants have been characterized in detail. The effects of varying the number (n) of 15-base-pair repeats and the recombination capabilities of the phage and host have been investigated. In these crosses, unequal crossovers can occur, yielding inserts different in size from the parental inserts. When the number of 15-mers is large (n = 12 or 20), most of the unequal crossovers have occurred within the 15-mers, resulting in an altered n value, although other homologies within the 5 S DNA sequence can also support unequal events. Increasing n in the parental inserts modestly increases the overall frequency of recombination and the percentage of altered inserts. We conclude that, in a bacterial setting, the 15-base-pair repeats stimulate recombination only slightly by allowing alternative registers for heteroduplex formation. The degree of stimulation observed is less than predicted by one simple model.

Phasmids: hybrids between ColE1 plasmids and E. coli bacteriophage lambda

Plasmids carrying cloned lambda att sites may be integrated into the bacteriophage genome by the site-specific recombination mechanism of lambda. The cross, referred to as "lifting" the plasmid, requires mixed infection of an Escherichia coli strain carrying the plasmid with two appropriately constructed "lifting" lambda phages. One phage donates a short left arm and the other donates a short right arm. These two short arms are of insufficient length to produce a viable phage genome and yield no recombinants when crossed on standard bacteria. However, viable recombinants are obtained when the genome length is extended by integration of one or more plasmids. We call these recombinants phasmids. They contain multiple att sites introduced at the ends of the integrated plasmids, and in the presence of integrase, recombination between these att sites can be exploited to effect release of the plasmid components. These novel genetic elements can be used in a variety of ways as vectors in genetic manipulation experiments. Sequences cloned in phasmids may be studied as a component of either a plasmid and or of a phage, and easily interconverted between the two states.

Sequence organization of the origins of DNA replication in lambdoid coliphages

We have determined the sequences of the ori region DNA of several phage lambda mutants and hybrids, which shed light on the mechanism of DNA replication in the lambdoid phages. These include the heterologous substitution hybrids lambda rep82:lambda and lambda rep80:lambda, a pseudorevertant of the ori-r93 mutant lambda r93hot5, and the insertion mutant lambda pk35. The ori regions of the three lambdoid phages, lambda, phi 80 and 82, all have repeated sequences, termed iterons, and A . T-rich zones. We note that a similar arrangement of DNA is also found in several other prokaryotic origins of replication. lambda and phi 80 have four iterons, and 82 has five. The origin of lambda r93hot5 is unusual in that contains only three iterons, yet the phage grows normally. Analysis of this mutant indicates that the spacing of iterons is crucial to ori function, whereas their number is not. This argues against the cloverleaf model for lambda ori structure (Hobom et al., 1979). In lambda pk35 the drug resistance element Tn903 is inserted into the "inceptor" (ice) site, proposed to be crucial for lambda replication initiation (Hobom et al., 1979); yet this phage grows normally.

Recombination of a eukaryotic DNA in bacteria

Single, 824 bp repeating units of xenopus laevis oocyte-type 5S DNA were inserted into the recombination vectors, lambda rva and lambda rvb. When the inserts had the same orientation with respect to the lambda chromosomes. Spi- imm434 recombinants were recovered by selection on a P2, lambda double lysogenic host. Because of the structure of the vectors, the crossover point in each recombinant must lie completely within the 5S DNA insert. The physical characteristics of these recombinants were determined by examination of restriction enzyme digests. By use of RecA mutant hosts and the Red- vector, lambda rvc, recombination frequencies were measured separately for the bacterial and phage systems. Some of the recombination events resulted in 5S DNA inserts of altered length due to unequal crossovers within repeated sequences in the 5S DNA spacer. The occurrence of just such events in frog 5S DNA had been predicted, based on the structure of 5S DNA and evolutionary considerations.

Cloning of the replication gene O of E. coli bacteriophage lambda and its expression under the control of the lac promoter

The expression of the replication gene O of bacteriophage lambda was put under the control of the lac promoter-operator region integrated into the pBR322 cloning vehicle. The new plasmid pKK104 was introduced into minicells and the O gene induced by isopropyl-beta-thiogalactoside (IPTG). The O protein could be identified as a major component in extracts from these cells, in association with the cell membrane fractions. The molecular weight of the O protein in SDS gels is about 33 000, and it is metabolically unstable but apparently stable upon isolation as a membrane-associated fraction.

Conditionally lethal amber mutations in the dnaA region of the Escherichia coli chromosome that affect chromosome replication

Three amber mutations, dna-801, dna-803, and dna-806, were isolated by localized mutagenesis of the dnaA-oriC region of the chromosome from an Escherichia coli strain carrying temperature-sensitive amber suppressors. When the mutations were not suppressed at 42 degrees C, the cells did not grow and DNA synthesis was arrested. They were very closely linked to each other and to the dnaA46 mutation. The mutant phenotype of each strain was converted to the wild type by infecting the mutants with specialized transducing phase lambda i21 dnaA-2 but not with lambda i21 tna. Derivatives of lambda i21 dnaA-2, each of which carried the amber mutation dna-801 dna-803, or dna-806, converted the dnaA mutant phenotype to Dna+ but did not convert rhe amber mutants to the wild-type phenotype. E. coli uvrB cells were irradiated with ultraviolet light and infected with each of these phage strains. An analysis of proteins synthesized in the cells revealed that two proteins with molecular weights of 50,000 and 43,000 were specified by lambda i21 dnaA-2 but not by lambda i21 tna. When the ultraviolet-irradiated cells did not carry an amber suppressor, the derivative phage with the amber mutation invariably failed to produce the 43,000-dalton protein, but when the host cell carried supF (tyrT), the protein was produced. The 50,000-dalton protein was unaffected.

T4 DNA topoisomerase: a new ATP-dependent enzyme essential for initiation of T4 bacteriophage DNA replication

A novel ATP-dependent DNA topoisomerase which makes reversible double-strand breaks in the DNA double helix has been purified to near homogeneity from T4 bacteriophage-infected Escherichia coli cells. Genetic data suggest that this activity is essential for initiating T4 DNA replication forks in vivo.

A coliphage lambda vector with enhanced biological containment: lambda gtALO.lambda B

The biological containment of the lambda gt family of cloning vectors has been enhanced by conditionally blocking DNA replication as well as head and tail morphogenesis. The vector, lambda gtALO.lambda B, was constructed by crossing the Oam29, Aama1 and Lam439 mutations into lambda gt.lambda B. The mutation blocking phage DNA replication, Oam29, is suppressed by suII+ or suIII+. The head gene mutation, Aama1, is suppressed by suIII+ but not by suII+ and the tail gene mutation, Lam439, is suppressed by suII+ but not by suIII+. This allows the option of increasing the biological containment by producing heads when a large amount of cloned DNA is being prepared from an individual isolate. A model recombinant, lambda gt Aama1 Lam439 Oam29.KmR' (lambda gtALO.KmR') was constructed and the containment of the vector was evaluated by the series of standardized experiments required for EK2 certification.

The nucleotide sequence of the region surrounding the replication origin of an R100 resistance factor derivative

The replication origin of a group of small plasmids derived from R100 was previously determined by electron microscopy (Ohtsubo et al., 1977). This region was subjected to extensive restriction enzyme analysis and the nucleotide sequence of the region containing the replication origin was determined using the Maxam and Gilbert sequencing technique. Various characteristics of this sequence, including a very interesting secondary structure are described and discussed.

Cloning of the replication gene P of bacteriophage lambda: effects of increased P-protein synthesis on cellular and phage DNA replication

A restriction fragment of lambdaDNA carrying the P gene was cloned in the high copy number plasmid RSF2124. Cells harbouring this new plasmid RSF2124/lambdaE complement lambdaPam80 phage. A lac promoter-operator region (lacP), produced by EcoRI digestion of plasmid pKB252, was inserted into RSF2124/lambdaE such that induction of the lac promoter by IPTG or lactose leads to increased production of the P gene product. A high amount of P protein in E. coli cells results in a slow inhibition of bacterial DNA synthesis, suggesting that the initiation reaction is blocked by P protein. Synthesis of lambdaDNA proceeds normally under these conditions. Nonsuppressing groPA15 mutant bacteria which are unable to support the replication of wild-type lambda (lambdawt), acquire the ability to replicate lambdaPam80 phage but not lambdawt when they are transformed with a plasmid carrying the lambdaP gene. When harbouring a plasmid containing the mutant Pamber 80 gene, groPA15 mutants are able to support the replication of lambdawt phage when infected at a high multiplicity. lambdaPam80 phage does not multiply in these cells.

DNA sequences and structural homologies of the replication origins of lambdoid bacteriophages

The DNA sequences for the origins of replication of the lambdoid bacteriophages phi80, 434, phi21, and lambdaimm21 (identical to phi21) have been determined and compared to the lambda structure. Two presumptive elaborate binding sites for two initiator proteins have been identified in their outer sections, while a replicational primer start site seems to be located in their centres.

Nucleotide sequence of the O gene and of the origin of replication in bacteriophage lambda DNA

The nucleotide sequence of the O gene in bacteriophage lambda DNA is presented. According to two possible initiator codons, the primary structure of the O protein deduced from the DNA sequence consists of 278 or 299 amino acid residues. Structure and function of the O protein--one of the two phage initiator proteins for lambda DNA replication--are discussed in the light of a secondary structure model for the O protein. The central part of the O gene contains a cluster of symmetrical sequences extending over 160 base pairs. The point mutation of the cis-dominant replication mutant ti12 is located in this region.

Control of lambda repressor prophage and establishment transcription by the product of gene tof

Control of expression of the bacteriophage lambda (lambda) repressor was studied by measuring repressor transcription in noninduced and derepressed lambda lysogens. Three distinct modes of leftward transcription were observed from cI and the adjacent genes associated with the control of repressor synthesis: The prophage or maintenance mode Prm-cI-rex-ti repressor transcript occurs from repressed lysogens; the oop (Po-oop-to) transcript, and the lit (lit-ti) RNA, from the distal half of gene rex, both occur from induced tof+ prophage; the repressor establishment mode of transcription is observed throughout the rex-cI-tof-y-cII-oop interval between Po and ti from induced tof- prophage. The overall level of establishment mRNA synthesis is partially template dependent. However, the actual initiation step for repressor establishment transcription requires the participation of the lambda cIII, cII products, and also either requires the activity of Escherichia coli replication proteins, or is triggered by a replication initiation event. The cII cIII products do not positively stimulate de novo initiation of establishment transcription, but rather act after an initial replication-dependent step. Initiation of the establishment mode of repressor transcription is totally inhibited by more than 125-fold, in an all or none fashion, by the lambda antirepressor (Tof), the product of gene tof (cro). Since Tof only reduces the in vivo rightward transcription of cII from Pr by about 2-fold, we suggest that Tof inhibits repressor establishment transcription by either uncoupling the replication and cII-cIII dependent events, or by inhibiting the activity rather than the expression of the cIII, cII products. Our results do not fully support either of the present hypotheses that establishment transcription is initiated from the hypothetical Pre promoter in the y-interval, or arises through antitermination of the oop RNA. Since the initiation and control of the establishment mode of repressor transcription parallels the control of lit RNA synthesis, we propose a common mechanism underlies the initiation of these transcripts.

A thermoinducible lambda phage-ColE1 plasmid chimera for the overproduction of gene products from cloned DNA segments

Two segments of lambda have been cloned into the multicopy plasmid pBR322. One extends from N through cII (NcII segment, from 71.3 to 81.0% on the physical map) and the other from N through P (NOP segment, from 71.3 to 86.5% on the physical map). Cells carrying these recombinant plasmids express lambda immunity (cIts) and Rex function. In addition, they decrease the efficiency of plating at 32 degrees C of lambdavir and lambdaimm434, but not that of lambdaimm21. Recombinant plasmids with lambdaNOP segments (pKC14, pKC16) differ from recombinant plasmid with labmdaNcII segment (pKC10) in two respects: (i) strains carrying pKC14 or pKC16 are killed at 42 degrees C, and (ii) these strains are thermally inducible for plasmid DNA synthesis, resulting in increase of plasmid copy number from an uninduced level of 50 to more than 130 per chromosome. It was suggested that both these differences are related to functions contained in the lambda DNA segment extending from 81.0 to 86.5%. The usefulness of plasmid pKC16 for overproduction of gene products from cloned DNA segments was demonstrated by cloning the E. coli exonuclease III gene (xth) in pKC16. Thermal induction of this xth plasmid (pSGr) results in a 125-fold increase in exonuclease III activity over that of a control strain lacking the xth gene insert. The extent of exonuclease III overproduction obtained by cloning xth gene in a lambda vector was similar to that obtained with pSGR3.

Physical structure of the replication origin of bacteriophage lambda

The nucleotide sequence of part of the replication region of wild-type bacteriophage lambda and of four mutants defective in the origin of DNA replication (ori-) has been determined. Three of the ori- mutations are small deletions, and one is a transversion. The sequence of the origin region, defined by these mutations, contains a number of unusual features.

Construction of chimeric phages and plasmids containing the origin of replication of bacteriophage lambda

Segments of the replication control region of bacteriophage lambda (lambda) and lambda mutants defective in replication were attached in vitro to the phi80 phage vector Charon 3 and to the plasmid vector mini Col El (pVH51). The chimeric phages and plasmids have been used to localize the origin of lambda DNA replication and to facilitate a structural analysis of the lambda replicator.