Analysis of the traLEKBP sequence and the TraP protein from three F-like plasmids: F, R100-1 and ColB2.

The sequence of a region of the F plasmid containing the traLEKBP genes involved in plasmid transfer was compared to the equivalent regions of two IncFII plasmids, R100-1 and ColB2. The traLEK gene products of all three plasmids were virtually identical, with the most changes occurring in TraE. The TraB genes were also nearly identical except for an 11-codon extension at the 3' end of the R100-1 traB gene. The TraP protein of R100-l differed from those of F and ColB2 at its N terminus, while the ColB2 TraP protein contained a change of sequence in a predicted loop which was shown to be exposed in the periplasmic space by TnphoA mutagenesis. The effect of the altered TraP sequences was determined by complementing a traP mutant with clones expressing the traKBP genes of F, R100-1, and ColB2. The traP mutation in pOX38 (pOX38-traP474), a derivative of F, was found to have little effect on pilus production, pilus retraction, and filamentous phage growth and only a moderate effect on transfer. The transfer ability of pOX38-traP474 was shown to be affected by mutations in the rfa (lipopolysaccharide) locus and in ompA in the recipient cell in a manner similar to that for the wild-type pOX38-Km plasmid itself and could be complemented with the traP analogs from R100-1 and ColB2 to give an F-like phenotype. Thus, the TraP protein appears to play a minor role in conjugation and may interact with TraB, which varies in sequence along with TraP, in order to stabilize the proposed transmembrane complex formed by the tra operon products.     

Localization, cloning, and sequence determination of the conjugative plasmid ColB2 pilin gene.

ColB2 is a colicin-producing, 96-kilobase plasmid which encodes a conjugative system that is similar, but not identical, to F. A restriction map of this plasmid was generated, and DNA homology studies between F and ColB2 plasmids revealed homology only between their transfer operons. The locations of the ColB2 transfer operon and ColB2 pilin gene were localized on this restriction map. The gene encoding ColB2 pilin, traA, was cloned and sequenced. The pilin protein of ColB2 is identical to F, except at the amino terminus, where ala-gln of ColB2 pilin corresponds to Ala-Gly-Ser-Ser of F pilin. This is due to a 6-base-pair deletion in the ColB2 pilin gene. Biochemical studies on tryptic peptides derived from ColB2 pilin demonstrate the location of this gene to be correct. There is a putative signal peptidase cleavage site after the sequence Ala-Met-Ala, giving a signal peptide of 51 amino acids and a mature pilin protein of 68 amino acids (7,000 daltons). The amino terminus is blocked, probably with an acetyl group. A chimera containing the ColB2 pilin gene was able to complement an F traA mutant, demonstrating that the pilus assembly proteins of F can utilize the ColB2 pilin protein to form a pilus.     

Characterization and sequence analysis of pilin from F-like plasmids.

Conjugative pili are expressed by derepressed plasmids and initiate cell-to-cell contact during bacterial conjugation. They are also the site of attachment for pilus-specific phages (f1, f2, and QB). In this study, the number of pili per cell and their ability to retract in the presence of cyanide was estimated for 13 derepressed plasmids. Selected pilus types were further characterized for reactivity with anti-F and anti-ColB2 pilus antisera as well as two F pilus-specific monoclonal antibodies, one of which is specific for a sequence common to most F-like pilin types (JEL92) and one which is specific for the amino terminus of F pilin (JEL93). The pilin genes from eight of these plasmids were cloned and sequenced, and the results were compared with information on F, ColB2, and pED208 pilin. Six pilus groups were defined: I, was F-like [F, pED202(R386), ColV2-K94, and ColVBtrp]; IIA was ColB2-like in sequence but had a lowered sensitivity to f1 phage due to its decreased ability for pilus retraction [pED236(ColB2) and pED203(ColB4)]; IIB was ColB2-like but retained f1 sensitivity [pED200(R124) and pED207(R538-1)]; III contained R1-19, which had a ColB2-like amino terminus but had an additional lysine residue at its carboxy terminus which may affect its phage sensitivity pattern and its antigenicity; IV was R100-1-like [R100-1 and presumably pED241(R136) and pED204(R6)] which had a unique amino-terminal sequence combined with a carboxy terminus similar to that of F. pED208(Folac) formed group V, which was multipiliated and exhibited poor pilus retraction although it retained full sensitivity to f1 phage. The pED208 pilin gene could not be cloned at this time since it shared no homology with the pilin gene of the F plasmid.     

DNA sequence analysis of point mutations in traA, the F pilin gene, reveal two domains involved in F-specific bacteriophage attachment

Summary Six missense point mutations in traA (WPFL43,44,45,46,47 and 51), the gene encoding F pilin in the transfer region of the F plasmid, have been characterized for their effect on the transfer ability, bacteriophage (R17, QB and fl) sensitivity and levels of piliation expressed by the plasmid. The sequence analysis of the first five of these mutations revealed two domains in the F pilin subunit exposed on the surface of the F pilus which mediate phage attachment. These two domains include residues 14–17 (approximately) and the last few residues at the carboxy-terminus of the pilin protein. One of these mutants had a pleiotropic affect on pilus function and was thought to have affected pilus assembly. The sixthe point mutant (WPFL51), previously thought to be in traA, was complemented by chimeric plasmids carrying the traG gene of the F transfer region, which may be involved in the acetylation of the pilin subunit. A traA nonsense mutant (JCFL1) carried an amber mutation near the amino-terminus which is well suppressed in SuI⁺ (supD) and SuIII⁺ (supF) strains. Neither the antigenicity of the pilin nor the efficiency of plating of F-specific bacteriophages were affected when this plasmid was harbored by either suppressor strain. A second amber mutant (JCFL25) which is not suppressible, carried its mutation in the codon for the single tryptophan in F pilin, suggesting that this residue is important in subunit interactions during pilus assembly. Two other point mutants (JCFL32 and 44) carried missense mutations in the leader sequence (positions 9 and 13) which affected the number of pili per cell presumably by altering the processing of propilin to pilin.     

Origin of transfer of IncF plasmids and nucleotide sequences of the type II oriT, traM, and traY alleles from ColB4-K98 and the type IV traY allele from R100-1.

The complete nucleotide sequences of the ColB4-K98 (ColB4) plasmid transfer genes oriT, traM, and traY as well as the traY gene of R100-1 are presented and compared with the corresponding regions from the conjugative plasmids F, R1, and R100. The sequence encoding the oriT nick sites and surrounding inverted repeats identified in F was conserved in ColB4. The adenine-thymine-rich sequence following these nick sites was conserved in R1 and ColB4 but differed in F and R100, indicating that this region may serve as the recognition site for the traY protein. A series of direct repeats unique to the ColB4 plasmid was found in the region of dyad symmetry following this AT-rich region. This area also encodes 21-base-pair direct repeats which are homologous to those in F and R100. The traM gene product may bind in this region. Overlapping and following these repeats is the promoter(s) for the traM protein. The traM protein from ColB4 is similar to the equivalent products from F, R1, and R100. The traY protein from ColB4 is highly homologous to the R1 traY gene product, while the predicted R100-1 traY product differs at several positions. These differences presumably define the different alleles of traM and traY previously identified for IncF plasmids by genetic criteria. The translational start codons of the ColB4 and R100-1 traY genes are GUG and UUG, respectively, two examples of rare initiator codon usage.     

Major antigenic determinants of F and ColB2 pili.

F-like conjugative pili are expressed by plasmids with closely related transfer systems. They are tubular filaments that are composed of repeating pilin subunits arranged in a helical array. Both F and ColB2 pilin have nearly identical protein sequences, and both contain an acetylated amino-terminal alanine residue. However, they differ by a few amino acid residues at their amino termini. Rabbit antisera raised against purified F and ColB2 pili are immunologically cross-reactive by only 25%, as measured by a competition enzyme-linked immunosorbent assay (ELISA). A tryptic peptide corresponding to the first 15 amino acid residues of ColB2 pilin was isolated and found to remove nearly 80% of ColB2 pilus-directed rabbit antibodies. The corresponding tryptic peptide from F pilin, which reacted with anti-F pilus antibodies to remove 80%, was less than 20% reactive with anti-ColB2 pilus antiserum. Cleavage of these peptides with cyanogen bromide (at a methionine residue approximately in the middle of the peptide) did not affect the antigenicity of these peptides. Synthetic N alpha-acetylated peptides corresponding to the first eight amino acids of F pilin (Ac-Ala-Gly-Ser-Ser-Gly-Gln-Asp-Leu-COOH) and the first six amino acids of ColB2 pilin (Ac-Ala-Gln-Gly-Gln-Asp-Leu-COOH) were prepared and tested by competition ELISA with homologous and heterologous anti-pilus antisera. The F peptide F(1-8) inhibited the interaction of F pili and anti-F pilus antiserum to 80%, while the ColB2 peptide ColB2(1-6) inhibited anti-ColB2 pilus antiserum reacting with ColB2 pili by greater than 60%. The two peptides F(1-8) and ColB2(1-6) were inactive by competition ELISAs with heterologous antisera. These results suggest that the major antigenic determinant of both F and ColB2 pili is at the amino terminus of the pilin subunit and that 80% of antibodies raised against these pili are specific for this region of the pilin molecule.     

IncP plasmids are most effective in mediating conjugation between Escherichia coli and streptomycetes

Mobilizable shuttle plasmids containing the origin of transfer (oriT) region of plasmid F (IncFI), ColIb-P9 (IncI1), and RP4/RP1 (IncPα) were constructed to test the ability of the cognate conjugation system to mediate gene transfer from Escherichia coli to Streptomyces. The conjugative system of the IncPα plasmids was shown to be most effective in conjugative transfer, giving peak values of (2.7 ± 0.2) × 10⁻² S. lividans TK24 exconjugants per recipient cell. To assess whether the mating-pair formation system or the DNA-processing apparatus of the IncPα plasmids is crucial in conjugative transfer, an assay with an IncQ-based mobilizable plasmid (RSF1010) specifying its own DNA-processing system was developed. Only the IncPα plasmid mobilized the construct to S. lividans indicating that the mating-pair formation system is primarly responsible for the promiscuous transfer of the plasmids between E. coli and Streptomyces. Dynamic of conjugative transfer from E. coli to S. lividans was investigated and exconjugants starting from the first hour of mating were obtained. The text was submitted by the authors in English.     

Nature of the Carbohydrate and Phosphate Associated with ColB2 and EDP208 Pilin

Studies were carried out to elucidate the nature of phosphate and sugar linkages to F-like conjugative pili encoded by the ColB2Fdr (compatibility group FII) and EDP208 (compatibility group FV) plasmids. Both types of pili, when in the intact undissociated state, were found to contain approximately 3 mol of phosphate and 3 mol of sugar per mol of pilin. However, further purification of the two types of pilin by gel filtration chromatography in the presence of sodium dodecyl sulfate (SDS) removed all of the carbohydrate from EDP208 pilin and approximately 65% of the carbohydrate from ColB2 pilin. Approximately 0.8 to 1.0 mol of glucose per mol of protein remained associated with ColB2 pilin after SDS gel filtration chromatography, but it was not possible to determine whether this was covalently linked to the pilin, or tightly associated in an SDS-resistant manner. SDS-gel chromatography did not remove phosphate from either ColB2 or EDP208 pilins. ³¹P nuclear magnetic resonance studies indicated that the pilin-associated phosphate is involved in a phosphodiester linkage. Acetone precipitation or chloroform-methanol extraction of the purified pilin material reduced the phosphate associated with EDP208 pilin to less than 0.04 molecule per pilin monomer. ColB2 pilin, under the same conditions, retained approximately 0.5 phosphate per pilin monomer. The extracted phosphate-containing moieties were identified as phosphatidylglycerol and phosphatidylethanolamine by thin-layer chromatography. Since the ³¹P nuclear magnetic resonance spectra for both ColB2 and EDP208 were identical and no signal other than that of a phosphodiester was detected in the ColB2 spectrum, the phosphate remaining with the ColB2 pilin after chloroform-methanol extraction is most likely due to a tightly bound noncovalent residue.     

Promiscuous DNA transfer system of Agrobacterium tumefaciens: role of the virB operon in sex pilus assembly and synthesis

Conjugative transfer of DNA that occurs between bacteria also operates between bacteria and higher organisms. The transfer of DNA between Gram-negative bacteria requires initial contact by a sex pilus followed by DNA traversing four membranes (donor plus recipient) using a transmembrane pore. Accumulating evidence suggests that transfer of the T-DNA from Agrobacterium tumefaciens to plants may also occur via a conjugative mechanism. The virB operon of the Ti plasmid exhibits close homologies to genes that are known to encode the pilin subunits and pilin assembly proteins. The proteins encoded by the PilW operon of IncW plasmid R388 share strong similarities (average similarity=50.8%) with VirB proteins. Similarly, the TraA, TraL and TraC proteins of IncF plasmid F have similarities to VirB2, VirB3 and VirB4 respectively (average similarity = 45.3%). VirB2 protein (12.3 kDa) contains a signal peptidase-I cleavage sequence that generates a polypeptide of 7.2 kDa. Likewise, the 12.8 kDa propilin protein TraA of plasmid F also possesses a peptidase-I cleavage site that generates the 7.2 kDa pilin structural protein. Similar amino acid sequences of the conjugative transfer genes of F, R388 as well as plasmid RP4 and the genes of the ptl operon of Bortedella pertussis suggest the existence of a superfamily of transmembrane proteins adapted to the promiscuous transfer of DNA-protein complexes.     

Five control systems preventing transfer of Escherichia coli K-12 sex factor F.

The transfer inhibition systems of 28 Fin+ plasmids have been characterized, using Flac mutants insensitive to inhibition by R100 or R62. All F-like plasmids (except R455) and one N group plasmid determined systems analogous to that of R100; this is designated the FinOP system. None of these plasmids could supply a FinP component of the transfer inhibitor able to replace that of F itself. In addition to the FinOP and R62 transfer inhibition systems described previously, new systems were encoded by the F-like plasmid R455, the I-like plasmid JR66a, and the group X plasmid R485. Besides inhibiting F transfer, JR66a also inhibited F pilus formation and surface exclusion, whereas R485 inhibited only pilus formation and R455 inhibited neither. All three R factors inhibited transfer of J-independent Flac elements, indicating that they act directly on one or more genes (or products) of the transfer operon, rather than directly via traJ. The tral products and transfer origin sequences of two Fin+ F-like plasmids, ColB2 and R124, appear to have similar specificities to those of F itself.     

Effect of lipopolysaccharide mutations on recipient ability of Salmonella typhimurium for incompatibility group H plasmids.

Previous investigations of the incompatibility group F, P, and I plasmid systems revealed the important role of the outer membrane components in the conjugal transfer of these plasmids. We have observed variability in transfer frequency of three incompatibility group H plasmids (IncHI1 plasmid R27, IncHI2 plasmid R478, and a Tn7 derivative of R27, pDT2454) upon transfer into various Salmonella typhimurium lipopolysaccharide (LPS) mutants derived from a common parental strain, SL1027. Recipients with truncated outer core via the rfaF LPS mutation increased the transfer frequency of the IncH plasmids by up to a factor of 10(3). Mutations which resulted in the truncation of the residues following 3-deoxy-D-manno-octulosonic acid, such as the rfaE and rfaD mutations, decreased the transfer frequency to undetectable levels. Addition of phosphorylethanolamine, a component of wild-type LPS, to the media decreased the frequency of transfer of R27 into wild-type and rfaF LPS mutant recipients tested. Reversing the direction of transfer, by mating LPS mutant donors with wild-type recipients, did not affect the frequency of transfer compared to the standard matings of wild-type donor with LPS mutant recipient. These findings demonstrate that conjugation interactions affected by LPS mutation are not specific for the recipient cell. Our results suggest that LPS mutation does not affect conjugation via altered pilus binding but affects some later steps in the conjugative process, and alteration of transfer frequency by O-phosphorylethanolamine and LPS truncation is due to charge-related interactions between the donor and recipient cell.     

The lipopolysaccharide of recipient cells is a specific receptor for PilV proteins, selected by shufflon DNA rearrangement, in liquid matings with donors bearing the R64 plasmid

Shufflon DNA rearrangement selects one of seven PilV proteins with different C-terminal segments, which then becomes a minor component of the thin pili of Escherichia coli strains bearing the plasmid R64. The PilV proteins determine the recipient specificity in liquid matings. A recipient Escherichia coli K-12 strain was specifically recognized by the PilVA′, -C, and -C′ proteins, while E. coli B was recognized only by the PilVA′ protein. To identify specific PilV receptors in the recipient bacterial cells, R64 liquid matings were performed using various E. coli K-12 waa (rfa) mutants and E. coli B transformants as recipient cells. E. coli K-12 waa mutants lack receptors for specific PilV proteins. E. coli B cells carrying waaJ or waaJKL genes of E. coli K-12 were recognized by donors expressing the PilVC′ protein or the PilVC and -C′ proteins, respectively, in addition to the PilVA′ protein. Addition of E. coli K-12 or B lipopolysaccharide (LPS) specifically inhibited liquid matings. We conclude that the PilV proteins of the thin pili of R64-bearing donors recognize LPS molecules located on the surface of various recipient bacterial cells in liquid matings. Received: 2 September 1999 / Accepted: 18 November 1999     

Genetic Analysis of the Role of the Transfer Gene, traN, of the F and R100-1 Plasmids in Mating Pair Stabilization during Conjugation

Mating pair stabilization occurs during conjugative DNA transfer whereby the donor and recipient cells form a tight junction which requires pili as well as TraN and TraG in the donor cell. The role of the outer membrane protein, TraN, during conjugative transfer was examined by introduction of a chloramphenicol resistance cassette into the traN gene on an F plasmid derivative, pOX38, to produce pOX38N1::CAT. pOX38N1::CAT was greatly reduced in its ability to transfer DNA, indicating that TraN plays a greater role in conjugation than previously thought. F and R100-1 traN were capable of complementing pOX38N1::CAT transfer equally well when wild-type recipients were used. F traN, but not R100-1 traN, supported a much lower level of transfer when there was an ompA mutation or lipopolysaccharide (LPS) deficiency in the recipient cell, suggesting receptor specificity. The R100-1 traN gene was sequenced, and the gene product was found to exhibit 82.3% overall similarity with F TraN. The differences were mainly located within a central region of the proteins (amino acids 162 to 333 of F and 162 to 348 of R100-1). Deletion analysis of F traN suggested that this central portion might be responsible for the receptor specificity displayed by TraN. TraN was not responsible for TraT-dependent surface exclusion. Thus, TraN, and not the F pilus, appears to interact with OmpA and LPS moieties during conjugation, resulting in mating pair stabilization, the first step in efficient mobilization of DNA.     

The role of lipopolysaccharide structure in the recipient cell during plasmid-mediated bacterial conjugation

The role of the lipopolysaccharide (LPS) structure in the recipient cell during bacterial conjugation was studied using a series of well-defined LPS mutations in Salmonella minnesota. The plasmids Flac (IncFI) and R1drd19 (IncFII) transferred at a high frequency to the smooth S218 parent strain and the rough LPS mutants. However, R64drd1 1 (IncI alpha) transferred poorly to the LPS mutants compared with transfer to the smooth LPS parent strain. The decrease in R64drd1 1 transfer frequency correlated with the extent of the defect in LPS structure, suggesting that intact LPS on the recipient is a major requirement for R64drd1 1 mating. Transfer of the P-group plasmid, RK2, was not affected by changes in LPS structure. These results show that plasmids use different cell surface structures during conjugation, and that LPS is particularly important for R64drd1 1 transfer.     

IncP plasmids are most effective in mediating conjugation between Escherichia coli and streptomycetes

Mobilizable shuttle plasmids containing the origin of transfer (oriT) region of plasmid F (IncFI), ColIb-P9 (IncI1), and RP4/RP1 (IncPalpha) were constructed to test the ability of the cognate conjugation system to mediate gene transfer from Escherichia coli to Streptomyces. The conjugative system of the IncPalpha plasmids was shown to be most effective in conjugative transfer, giving peak values of (2.7 +/- 0.2) x 10(-2) S. lividans TK24 exconjugants per recipient cell. To assess whether the mating-pair formation system or the DNA-processing apparatus of the IncPalpha plasmids is crucial in conjugative transfer, an assay with an IncQ-based mobilizable plasmid (RSF1010) specifying its own DNA-processing system was developed. Only the IncPalpha plasmid mobilized the construct to S. lividans indicating that the mating-pair formation system is primarly responsible for the promiscuous transfer of the plasmids between E. coli and Streptomyces. Dynamic of conjugative transfer from E. coli to S. lividans was investigated and exconjugants starting from the first hour of mating were obtained.     

Chemical Analysis of Lipopolysaccharides of Shigella sonnei Form II Strains Expressed by Cloned Form I Antigen Genes

A compositional sugar analysis was carried out on lipopolysaccharide (LPS) from Shigella sonnei form II in which a plasmid with cloned form I antigen genes had been introduced. The recipient form II strains contained galactose, glucose, heptose, glucosamine, and 2-keto-3-deoxyoctonic acid (KDO) (2: 3: 1: 2: 2) in its LPS, while the transformant form I LPS contained, besides these sugars, N-acetyl-L -altrosaminouronic acid as an additional sugar constituent, which is known to be one of the antigenic determinants of form I antigen.     

Temperature-sensitive,lipopolysaccharide-deficient mutants of Salmonella typhimurium

Two mutants of Salmonella typhimurium LT2, which were temperature-sensitive for lipopolysaccharide (LPS) synthesis, were isolated from a galE ^- strain based on their resistance to phage C21 and sensitivity to sodium deoxycholate at 42°C. They produced LPS of chemotype Rc at 30°C and deep-rough LPS at 42°C. P22-mediated transductional analysis showed that the mutations responsible for temperature sensitivity are located in the rfa cluster where several genes involved in the synthesis of the LPS core are mapped. A plasmid, carrying rfaC, D and F genes of Escherichia coli K-12, complemented these mutations. These genes are responsible for the synthesis of the inner-core region of the LPS molecule. This indicates that genetic defects in these temperature-sensitive mutants affect the inner-core region of LPS.     

Location of the antigenic determinants of conjugative F-like pili

The amino terminus of the pilin protein constitutes the major epitope of F-like conjugative pili studied to date (F, ColB2, R1-19, R100-1, and pED208). Anti-pED208 pilus antibodies were passed through a CNBr-Sepharose affinity column linked to bovine serum albumin which was conjugated to a synthetic peptide, AcP(1-12), containing the major epitope at the amino terminus of pED208 pilin. This allowed the separation of two classes of antibodies; one was specific for the amino terminus and bound to the column, while the other, which recognizes a second epitope on the pilus, did not bind to the column. In addition, antibodies were raised against two amino-terminal peptide-bovine serum albumin conjugates [AcP(1-8) and AcP(1-12)] to ensure a source of pure, high-titer antibodies directed against the amino terminus. The location of these antibodies on intact pili was assayed by immunoelectron microscopy with a protein A-gold technique. The amino terminus-specific antibodies did not bind to the sides of the pili but appeared to be associated with the pilus tip. In addition, these antibodies were found to bind to the vesicle-like structure at the base of the pilus. The anti-pilus antibodies not specific for the amino terminus (unbound immunoglobulin G) were found to bind to the sides of the pilus. Anti-F and anti-ColB2 pilus antibodies bound to the sides of F, ColB2, and R1-19 pili, which have only their secondary epitope in common. The carboxyl-terminal lysine of R1-19 pilin prevents the absorption of anti-F plus antiserum but not anti-ColB2 pilus antiserum to the sides of the pilus, presumably by interfering with the recognition of this secondary epitope.     

Comparison of Proteins Involved in Pilus Synthesis and Mating Pair Stabilization from the Related Plasmids F and R100-1: Insights into the Mechanism of Conjugation

F and R100-1 are closely related, derepressed, conjugative plasmids from the IncFI and IncFII incompatibility groups, respectively. Heteroduplex mapping and genetic analyses have revealed that the transfer regions are extremely similar between the two plasmids. Plasmid specificity can occur at the level of relaxosome formation, regulation, and surface exclusion between the two transfer systems. There are also differences in pilus serology, pilus-specific phage sensitivity, and requirements for OmpA and lipopolysaccharide components in the recipient cell. These phenotypic differences were exploited in this study to yield new information about the mechanism of pilus synthesis, mating pair stabilization, and surface and/or entry exclusion, which are collectively involved in mating pair formation (Mpf). The sequence of the remainder of the transfer region of R100-1 (trbA to traS) has been completed, and the complete sequence is compared to that of F. The differences between the two transfer regions include insertions and deletions, gene duplications, and mosaicism within genes, although the genes essential for Mpf are conserved in both plasmids. F+ cells carrying defined mutations in each of the Mpf genes were complemented with the homologous genes from R100-1. Our results indicate that the specificity in recipient cell recognition and entry exclusion are mediated by TraN and TraG, respectively, and not by the pilus.     

Plasmid control of recombination in E. coli K 12

Summary The recombination proficiency of three recipient strains of Escherichia coli K 12 carrying different plasmids was investigated by conjugal mating with Hfr Cavalli. Some plasmids (e.g. R1drd 19, R6K) caused a marked reduction in the yield of recombinants formed in crosses with Hfr but did not reduce the ability of host strains to accept plasmid F104. The effect of plasmids on recombination was host-dependent. In Hfr crosses with AB1157 (R1-19) used as a recipient the linkage between selected and unselected proximal markers of the donor was sharply decreased. Plasmid R1-19 also decreased the yield of recombinants formed by recF, recL, and recB recC sbcA mutants, showed no effect on the recombination proficiency of recB recC sbcB mutant, and increased the recombination proficiency of recB, recB recC sbcB recF, and recB recC sbcB recL mutants. An ATP-dependent exonuclease activity was found in all tested recB recC mutants carrying plasmid R1-19, while this plasmid did not affect the activity of exonuclease I in strain AB1157 and its rec ^– derivatives. The same plasmid was also found to protect different rec ^– derivatives of the strain AB1157 against the lethal action of UV light. We suppose that a new ATP-dependent exonuclease determined by R1-19 plays a role in both repair and recombination of the host through the substitution of or competition with the exoV coded for by the genes recB and recC.