Further characterization of the F fertility inhibition systems of "unusual" Fin+ plasmids.

Flac mutants insensitive to transfer inhibition by R factors. JR66a and R485 were isolated and characterized. Representative mutations were cis dominant and are therefore presumed to be at the sites of action, fisU and fisV, respectively, of the FinU and FinV transfer inhibition systems encoded by JR66a and R485. The mutants were used to confirm that the FinU and FinV fertility inhibition systems are different from each other and from the FinOP, FinQ, and FinW systems of R100, R62, and R455, respectively. Together with traO and fisQ mutants of Flac, the new mutants were also used to investigate the nature of the F fertility inhibition systems encoded by a further group of "unusual" Fin+ plasmids. Of these, two incompatibility group X plasmids were found to carry finO+ genes, and of five incompatibility group I plasmids, three encoded FinQ systems, one the FinU system, and one a new system (FinR). Transfer of a variety of derepressed F-like plasmids was inhibited by the FinQ, FinU, and FinV systems, but a quantitatively very different levels; this emphasizes the differences as well as the similarities between the conjugation systems of F-like plasmids.     

Inhibition of Flac Transfer by the Fin+ I-Like Plasmid R62

Flac mutants have been isolated in Escherichia coli K-12 which carry dominant mutations resulting in insensitivity to transfer inhibition by the Fin(+) I-like plasmid R62. These mutants were still sensitive to transfer inhibition by the fin(+) F-like plasmid R100 and, conversely, FlactraO(-) and traP(-) mutants, which are insensitive to R100 inhibition, were still sensitive to R62. The sites of action of the two inhibition systems are therefore different. Furthermore, inhibition by R62, unlike R100, did not require an F-specified product. Like R100, R62 prevented transfer, pilus formation, and surface exclusion and, therefore, probably inhibits expression of the transfer operon traA through traI. However, R62 was different from R100 in inhibiting transfer of J-independent mutants, indicating that its effect on the transfer operon is probably direct rather than via traJ. This is consistent with the different sites of action of the two inhibition systems. None of the Flac mutants overproduced pili in the absence of R62, although one mutant differing from those described above showed increased levels of transfer and surface exclusion.     

Analysis and characterization of the IncFV plasmid pED208 transfer region

pED208 is a transfer-derepressed mutant of the IncFV plasmid, F(0)lac, which has an IS2 element inserted in its traY gene, resulting in constitutive overexpression of its transfer (tra) region. The pED208 transfer region, which encodes proteins responsible for pilus synthesis and conjugative plasmid transfer, was sequenced and found to be very similar to the F tra region in terms of its organization although most pED208 tra proteins share only about 45% amino acid identity. All the essential genes for F transfer had homologs within the pED208 transfer region with the exception of traQ, which encodes the chaperone for stable F-pilin expression. F(0)lac appears to have a fertility inhibition system different than the FinOP system of other F-like plasmids, and its transfer efficiency was increased in the presence of F or R100, suggesting that it could be mobilized by these plasmids. The F-like transfer systems specified by F, R100, and F(0)lac were highly specific for their cognate origins of transfer (oriT) as measured by their abilities to mobilize chimeric oriT-containing plasmids.     

Systems regulating the tra genes of derepressed F-like plasmids

A study was made of the ability of reference plasmids of the 6 known Fin-groups to inhibit the functions of transfer genes (tra-genes) of the 4 derepressed F-like plasmids (pAP22-2, pAP38, pAP43, pAP53). It was shown that unlike the derepressed Flac plasmid, the conjugation transfer of pAP38 and pAP53 plasmids was inhibited only by, the FinV plasmid, whereas pAP22-2 plasmids by Fin V and Fin V plasmids. The formation of donor-specific pili in case of pAP38 plasmid was inhibited by Fin Q, Fin U and Fin V plasmids, in case of pAP43 plasmid by Fin U Fin V and Fin W plasmids.     

Plasmid Specificity of The Origin of Transfer of Sex Factor F

The ability of F-like plasmids to promote transfer from the F origin of transfer was determined. Chromosome transfer was measured from plasmid derivatives of RecA(-) Hfr deletion strains which had lost all the F transfer genes but which in some cases retained, and in others had also lost, the origin sequence. ColV2 and ColVBtrp could initiate transfer from the F origin, but R100-1, R1-19, and R538-1 drd could not. These results can be correlated with the plasmid specificity of the traI components of the different plasmid transfer systems, supporting the hypothesis that the origin of transfer is the site of action of the traI product. Most F-like plasmids, including R1-19 and R538-1 drd, could transfer ColE1, consistent with previous findings that the (plasmid-specific) traI product is not necessary for ColE1 transfer by Flac; ColE1 transfer may be initiated by a ColE1-or host-determined product. R100-1 and R136fin(-) could not transfer ColE1 efficiently, apparently because of differences residing in their pilus-forming genes.     

Genetic regulation of the function of E. coli plasmid pAP42 transfer genes

Sensitivity of the genetic transfer system of F-like plasmid pAP42 marked with the transposons Tn1 and TN9 to fertility inhibitors of six reference Fin-groups was studied. It was shown that transfer function and donor-specific piliformation of the plasmid under study were inhibited by reference plasmids of FinU and FinV groups, surface exclusion by plasmids of FinU and FinQ groups. The different influence of the FinOP group plasmid on transfer functions of the marked plasmids pAP42::Tn1 and pAP42::Tn9 that is likely to be connected with the effect of incorporated transposons was determined.     

Derepression of F factor function in Salmonella typhimurium

In Salmonella typhimurium LT2 the F factor of Escherichia coli K-12 replicates normally but is repressed; Flac+ cells give no visible lysis on solid media with male-specific phages, low frequency transfer of Flac+ (0.001-0.007 per donor cell), few f2 infective centers (0.002-0.006 per cell), and they propagate male-specific phages to low titers. Thus they display a Fin+ (fertility inhibition) phenotype. This repression, owing to pSLT, a 60 Mdal plasmid normally resident in S. typhimurium, was circumvented by the following materials: (i) Flac+ plasmids from E. coli with mutations in finP or traO; (ii) a S. typhimurium line which had been cured of pSLT; (iii) pKZl, a KmR plasmid in the same Inc group as pSLT, which caused expulsion of pSLT and made Fin- lines; (iv) F-Fin- mutants which originated spontaneously and which are present in most Hfr strains of S. typhimurium. Strains which are derepressed for F function by the above methods give visible lysis on solid media with male-specific phages, ca. 1.0 Lac+ recombinants per donor cell in conjugal transfer, ca. 0.82 f2 infective centers per cell, over 80% of cells with visible F pili, and propagation of male-specific phages to high titer. These data confirm earlier observations that pSLT represses F by the FinOP system. In addition, it shows that there is no other mechanism which represses F function in S. typhimurium. If donor function is derepressed by one of the above methods, and if rough recipient strains are used, F-mediated conjugation in S. typhimurium LT2 is as efficient as in E. coli K-12.     

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.     

Complementation analysis of derepressed mutants of F-like plasmids of Escherichia coli

Complementation analysis of a number of conjugative transfer functions was performed in derepressed (drd) mutants of E. coli F-like plasmids. The major part of double plasmid complexes investigated has revealed the formation of complementation transfer inhibitor of Fin V-type, or less frequently--the formation of Fin U-type inhibitor. An additional complementation analysis of drd plasmids defective at Fin V region genes has demonstrated at least three genes (denoted A, B, C) in the structure of this region.     

The role of the pilus in recipient cell recognition during bacterial conjugation mediated by F-like plasmids

The effects of defined mutations In the lipopolysaccharide (LPS) and the outer membrane protein OmpA of the recipient cell on mating-pair formation in liquid media by the transfer systems of the F-Iike plasmids pOX38 (F), ColB2 and R100-1 were investigated. Transfer of all three plasmids was affected differently by mutations in the rfa (LPS) locus of the recipient cell, the F plasmid being most sensitive to mutations that affected rfaP gene expression which is responslbie for the addition of pyrophosphorylethanolamine (PPEA) to heptose I of the inner core of the LPS. CoIB2 transfer was more strongly affected by mutations in the heptose II-heptose III region of the LPS (rfaF) whereas R100-1 was not strongly affected by any of the rfa mutations tested. ompA but not rfa mutations further decreased the mating efficiency of an F plasmid carrying a mutation in the mating-pair stabilization protein TraN. An F derivative with a chloramphenicol acetyltransferase (CAT) cassette interrupting the traA pilin gene was constructed and pilin genes from F-like plasmids (F, ColB2, R100-1) were used to complement this mutation. Unexpectediy, the results suggested that the differences in the pilin sequences were not responsible for recognizing specific groups in the LPS, OmpA or the TraT surface exclusion protein. Other corroborating evidence is presented suggesting the presence of an adhesin at the F pilus tip.     

F factor conjugation is a true type IV secretion system

The F sex factor of Escherichia coli is a paradigm for bacterial conjugation and its transfer (tra) region represents a subset of the type IV secretion system (T4SS) family. The F tra region encodes eight of the 10 highly conserved (core) gene products of T4SS including TraAF (pilin), the TraBF, -KF (secretin-like), -VF (lipoprotein) and TraCF (NTPase), -EF, -LF and TraGF (N-terminal region) which correspond to TrbCP, -IP, -GP, -HP, -EP, -JP, DP and TrbLP, respectively, of the P-type T4SS exemplified by the IncP plasmid RP4. F lacks homologs of TrbBP (NTPase) and TrbFP but contains a cluster of genes encoding proteins essential for F conjugation (TraFF, -HF, -UF, -WF, the C-terminal region of TraGF, and TrbCF) that are hallmarks of F-like T4SS. These extra genes have been implicated in phenotypes that are characteristic of F-like systems including pilus retraction and mating pair stabilization. F-like T4SS systems have been found on many conjugative plasmids and in genetic islands on bacterial chromosomes. Although few systems have been studied in detail, F-like T4SS appear to be involved in the transfer of DNA only whereas P- and I-type systems appear to transport protein or nucleoprotein complexes. This review examines the similarities and differences among the T4SS, especially F- and P-like systems, and summarizes the properties of the F transfer region gene products.     

RP1 properties and fertility inhibition among P, N, W, and X incompatibility group plasmids.

Incompatibility group P plasmids demonstrate strong entry exclusion properties. Stringent incompatibility is also observed in the absence of entry exclusion. These observations have been facilitated by the study of a nontransmissible plasmid, RP1-S2, derived from RP1 by transductional shortening. RP1-S2 retains carbenicillin and tetracycline resistances as well as loci that cause either the loss of P plasmids (incp) or a locus specifying susceptibility to curing (sinp) in the presence of a P plasmid. RP1-S2 can be mobilized by an incompatibility group W plasmid, R388, and also freely forms recombinants with R388. P, N, and W incompatibility group plasmids all encode information for the receptor of the cell wall-adsorbing phage PRD1. Based on the premise that the location of this receptor is analogous to entry exclusion factors for F-like plasmids and hence a regulated transfer region determinant, we tested fertility inhibition relationships among these plasmid groups. We detected both reciprocal and nonreciprocal fertility inhibition relationships for bacteria containing various combinations of W, N, and P group plasmids. The nonreciprocal nature of some combinations, we believe, reflects the identity of the point mutation reading to derepression of the plasmid in question. Reciprocal fertility inhibition, on the other hand, may reflect the reconstruction of a fertility inhibition system through complementation. An X incompatibility group plasmid, known to affect the fertility of an N group plasmid, was also shown to inhibit P plasmid fertility. These observations may indicate a possible evolutionary relationship(s) of plasmids unrelated by the criteria of incompatibility, pilus phage specificity, or plasmid host range.     

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.     

Identification of a membrane protein associated with expression of the surface exclusion region of the F transfer operon.

Membrane preparations from radioactively labeled male and female strains of Escherichia coli K-12 were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. An intensely labeled band corresponding to a protein of molecular weight of 24,000 was readily apparent in preparations from Hfr and F-prime strains but not in those from female strains. When preparations from a series of Hfr strains containing transfer operon deletions were examined, presence of the band was found to be associated with retention of the region of the F transfer operon between ilzA and traD. Thus, the band ("protein S") appears to be the product of an F tra operon activity corresponding to traS (the gene for surface or entry exclusion), or an unknown gene in its vicinity. As predicted, protein S was subject to Fin+ control; only a faint band was detectable if the repressed plasmid R100 was also present in the F lac strain. A 24,000-dalton protein was also found in membrane preparations from strains carrying the derepressed plasmids R100-1 and R1-19 but not in those from strains carrying the repressed plasmids R100 or R1. Thus, the appearance of protein S in the membrane may be a general phenomenon resulting from transfer operon expression of F-like plasmids.     

Plasmid Specificity of Two Proteins required for Conjugation in <Emphasis Type="Italic">E. coli</Emphasis> K12

IN recent years, many episomes other than the F factor of E. coli K12 have been discovered, such as the colicinogenic factors and the drug resistance transfer factors. Some of these seem to be related to F since they determine a pilus similar to the F pilus in serological properties and male specific phage sensitivity¹. These F-like plasmids frequently inhibit their own transfer and of F when present in the same cell, but mutants have been obtained which do not produce the inhibiting substance and which allow transfer at a high rate^1,2. One such mutant, R100-1, was used in the work to be described here.     

Characterization of the Tra2 Region of the IncHI1 Plasmid R27

In this study, the DNA sequence of one of the transfer regions of the IncHI1 plasmid R27 was determined. This region, which corresponds to coordinates 0-40 on the R27 map has been called the Tra2 region, and is believed to be involved in mating pair formation. DNA sequence analysis of the transfer region identified 11 open reading frames which showed similarities to the transfer genes from other conjugative systems. The R27 transfer genes appear to most closely resemble the genes from the F plasmid and Sphingomonas aromaticivorans plasmid pNL1, both within the individual genes and in the overall gene order. The Tra2 region is also distinct in that replication, partitioning, and stability genes are found in the middle of the transfer region. The R27 Tra2 region also contains a gene, trhF, which appears to be related to the TraF genes of Agrobacterium and Rhizobium species. This, along with the temperature-sensitive transfer system found in both H plasmids and Agrobacterium, leads to the speculation that the R27 transfer region evolved from both ancestral F-like and P-like plasmids.     

Decreased conjugation efficiency by Fla-mutants of Escherichia coli K-12 bearing F-like plasmids.

Fla-, Pil-mutants of Escherichia coli K-12 were found to have decreased transfer efficiency of F-like resistance plasmids as compared to the parent strains. This was accompanied by decreased production of conjugation pili and decreased resistance level to some, but not all, of the antibiotics to which resistance was conferred. There was no reduction in pilus production or transfer efficiency in any of the mutants when the plasmid was F'gal. This host-mediated influence on conjugation pilus production is discussed with reference to a possible loss of cell envelope integrity which causes the simultaneous loss of all cellular appendage structures.