Specific piliation directed by a fertility factor and a resistance factor of Escherichia coli

Specificities of IncF plasmid conjugation genes

The conjugation regions of IncF plasmids are closely related in that they share extensive DNA homology, and that they specify related pili. Variations between individual conjugation gene products of different IncF plasmids have, however, been noted. We have extended these observations by carrying out a systematic survey of twelve such plasmids, to examine the numbers and the groupings of the plasmid-specific alleles of several genes required for conjugation and its control.

Using vector plasmids carrying cloned origins of transfer (oriT), four different specificities were recognized, and these were correlated with the specificities of the genes with products that may act at this site (traM, traYandtraZ). ThetraYgene is the first gene of the major transfer operon, and is therefore located close to the site at which thetraJprotein acts to induce expression of the operon: correspondingly, correlation was observed between theoriT/traMYZandtraJspecificities in most of the plasmids. In turn,traJis negatively regulated by thefinOandfinPproducts acting in concert: thefinOproduct was relatively non-specific, but sixfinPalleles were identified, again with specificities correlated with those oftraJ. Our explanation for this unexpectedly large number offinPalleles derives from the concept that thefinPproduct is an RNA molecule rather than a protein. Although the conjugative pili encoded by IncF plasmids are closely related, they confer different efficiencies of plating of the various F-specific bacteriophages. We distinguished four groups on this basis, presumably resulting from differences in the primary amino-acid sequences of the pilin proteins. These groups could be related to the surface exclusion system specificities, consistent with the hypothesis that surface exclusion acts at least in part by preventing interaction between the pilus and the recipient cell surface.

From these data, information about the evolutionary relationships between the twelve IncF plasmids can be deduced.

Use of a known plasmid and transposon as tracers for the incompatibility classification of a cryptic plasmid

We have developed a novel genetic technique by which an unknown plasmid can be classified by the use of plasmid of known incompatibility group. In phenocopy state, cells harboring plasmids of known incompatibility group behave as normal recipients and receive plasmids belonging to the same incompatibility group at a high frequency. This work provides additional evidence that plasmids in 'phenocopy' hosts do not replicate and therefore fail to demonstrate their incompatibility barrier to the incoming plasmids. A cryptic plasmid, now called pWS7, has been identified by this method in a pili, fla female strain of E. coli K12. Genetic analysis shows the plasmid pWS7 is in fact, a sex-factor which is curable with acridine orange. It belongs to the Inc F1 group. Physical analysis confirms its size to be 124 Kb. The plasmid has been labelled genetically with a transposon Tn903 in a recA host and further characterized by heteroduplex analysis. A DNA sequence homology between pWS7 and F'lac plasmid extends only in F-regions, 2.8F-94.5F. The pili, fla host strain of pWS7 shows a high frequency of transformation for recombinant DNA and rapid propagation for a male-specific RNA phage, R17.

R-plasmid transfer and its response to nalidixic acid

The conjugational transfer efficiency of 41 wild-type R-plasmids was studied in Escherichia coli K-12. Type I R-plasmids were transferred at comparatively high and rather uniform frequencies, whereas type F R-plasmids showed less uniform and, on average, somewhat lower transfer frequencies. R-plasmids not mediating sensitivity to F-, I-, or N-specific phages showed moderate transfer frequencies, and type N R-plasmids showed very low transfer frequencies. Various lines of evidence suggest that a well-expressed, but functionally inefficient, conjugation apparatus is the cause of the poor transfer of type N R-plasmids in liquid medium. Nalidixic acid efficiently inhibited transfer of type I and particularly type F R-plasmids, whereas the transfer of type N plasmids was resistant to the drug. Type F and type I plasmids appear to depend on at least one host function for their transfer, namely, the nalidixic acid-sensitive reaction in vegetative chromosome replication, whereas type N plasmids are independent of this function.

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.

Cloning, isolation, and characterization of replication regions of complex plasmid genomes

EcoRI endonuclease-generated DNA fragments carrying replication regions of the F'lac and R6-5 plasmids have been cloned and isolated, using as a selection vehicle a nonreplicating ampicillin-resistance DNA fragment derived from a Staphylococcus aureus plasmid. Heteroduplex analysis of the constructed plasmid chimeras and the parent replicons has localized the cloned R6-5 replication region to a DNA segment between kilobase pair coordinates 1.0 and 88.0 on the R6-5 map. Physical proximity between the plasmid replication functions and the locus governing plasmid incompatibility has been shown for both parent replicons. The cloning method reported appears to be generally applicable for the identification and isolation of replication regions of a variety of complex genomes.

Isolation and properties of mutants of the FP2 sex factor ofPseudomonas aeruginosa

Two new types of mutants of the FP2 sex factor have been isolated in males ofP. aeruginosastrain PAT. Males harbouring FPd mutants are unable to mediate either sex factor or host chromosome transfer, although they retain the exclusion and precipitation characteristics of wild-type males. Males harbouring the FPs mutant apparently have an altered cell surface as indicated by their loss of precipitability, and although their donor properties are similar to those of wild-type males they show a slightly reduced conjugal fertility.

A previously described sex factor mutant FP* (Stanisich & Holloway, 1972) can be transferred to males carrying either the FPs or FPd factors, and the heterozygous strains produced show restoration of wild-type properties, i.e. wild-type precipitation characteristics of the FPs mutants, and transferability of FPd mutants respectively. This suggests that the mutations of the FPs and FPd factors are recessive to the alleles carried by the FP* factor. The ability to produce such heterozygous strains supports the view that at least two copies of the FP2 factor occur in strain PAT males.

Transfer-positive J-independent revertants of the F factor inEscherichia coliK 12

Cells carryingtraJ−mutants of F are transfer-deficient and are good recipients in conjugation (Achtman, Willets & Clark, 1972). In addition, thetraJgene product is involved in pilus- and in plasmid-specificity (Willetts, 1971). J-independent mutants were isolated as revertants of atraJ−mutant; they still carry thetraJ−mutation but also carry at least one other mutation which results in transfer in the absence of thetraJgene product. J-independent transfer of these mutants is not inhibited by the R 100 repressor. Various models are presented which can account for the properties oftraJ−mutants and of these J-independent revertants.

Studies on sex pili: mutants of the sex factor F in Escherichia coli defective in bacteriophage-adsorbing function of F pili

Ultraviolet irradiation or nitrosoguanidine treatment of Escherichia coli K-12 strain JE3100 (F'(8)/fla pil) led to the isolation of six mutants defective in F pili function. The defects were shown to be caused by mutations in the F factor. The mutants retained conjugal fertility, although they were less efficient than parental F'(8) strain, and continued to synthesize F pili. Three of the mutants (strains KE196, 198, and 200) had lost sensitivity to male-specific MS2 phage, and the other three (strains KE161, 163, and 164) were insensitive to Qbeta and f1 as well as MS2 phages. F pili on strains KE196, 198, and 200 cells continued to adsorb MS2 phage, whereas those of strains KE161, 163, and 164 did not adsorb MS2 phage. The correlation of the mutant phenotypes with those of other F mutants reported in the literature is discussed.

The relationship between the transfer systems of some bacterial plasmids

The relationship between the transfer systems of several plasmids was investigated, using as criteria complementation of a series of transfer-deficientFlacmutants, the efficiency of plating of F-specific phages, and identification of surface exclusion systems. The transfer systems of ColV2 and ColVBtrpwere similar to that of F except for surface exclusion: ColVBtrpspecified a surface exclusion system different from that of F, and ColV2 did not specify any detectable system. The transfer system of R1–19 was also similar to that of Flac, but the products oftraA,traIandtraJwere plasmid-specific. R1–19 determined the same surface exclusion system as ColVBtrp, and this was under the control of the transfer inhibitor. The transfer systems of ColIbdrdandFlacwere unrelated. ColEl was transferred byFlac traI−mutants, but not by mutants in other cistrons.

Nonchromosomal antibiotic resistance in bacteria. V. Isolation and characterization of R factor mutants exhibiting temperature-sensitive repression of fertility

Several distinct types of derepressed R factor mutants have been isolated and identified following nitrosoguanidine mutagenesis. One class of mutants appears to synthesize a temperature-sensitive repressor substance, as shown by the effects of temperature on R factor transfer, on male-specific phage sensitivity, and on F factor and Hfr transfer. In addition, two classes of repressor-in-sensitive derepressed R factor mutants have been identified: although both inhibit F factor transfer at 42 and 32 C, one mutant (R6-5drd50) represses its own fertility at only the low temperature. These results imply the existance of a plasmid-specific component of the R factor repression system, in addition to a component which acts on both R and F factor fertility.

Specific role of sex pili in the effective eliminatory action of sodium dodecyl sulfate on sex and drug resistance factors in Escherichia coli

Evidence is presented for the specific role of sex pili in the eliminatory action of sodium dodecyl sulfate (SDS) on sex (F) and drug resistance (R) factors in Escherichia coli K-12 strains leading to their loss. SDS at 0.03% concentration lysed JE3100 F(8) (+) (F-gal)/gal(-)fla(-)pil(-) in Penassay broth after they had grown exponentially and reached maximum growth to the extent that the agent at concentrations higher than 1% did. However, the agent was only effective in eliminating sex factors from JE3100 in high frequencies at concentrations higher than 1%. Increase of osmotic pressure of the culture with SDS at concentrations as low as 0.03 to 0.1% by addition of sucrose led to the substantial increase of elimination efficiency. Reconstruction experiments between F(8) (+) and F(-) cells in the SDS culture revealed the selective growth of F(-) cells as well as a delay of maximum growth of F(-) variants derived from F(8) (+) cells, compared with those of F(8) (+) cells, as well as F(-) cells originally added to the culture. The agent was not very effective in eliminating sex factors from JE3427 F(8)m(+)5/fla(-)pil(-) cells which lack the function of production of F pili. F(8)m(+)5 cells showed a sensitivity toward SDS intermediate between those of F(8) (+) and F(-) cells. SDS was further effective in eliminating R factors from KE132 R(100-1) (+)/fla(-)pil(-) cells in high efficiency; however, the action was not efficient with KE133 F(100) (+) cells possibly with fewer sex pili than R(100-1) (+). Action of acridine orange on these F(+) or R(+) strains was found to be different in some aspects from that of SDS.

Inhibition of gene transfer by antiserum and identification of serotypes of sex pili

Gene transfer by conjugation due to F or R (drug resistance) factors is inhibited by antibody to the sex pili. Serological analysis is able to distinguish between the sex pili determined by closely related sex factors, and the specificity of inhibition of transfer agrees with that previously determined by direct electron microscopical observation of antibody bound to the sex pili (10). Inhibition of transfer can therefore be applied to the identification of wild-type R factors with repressed sex factors that determine too few pili to be examined directly. It can also be used to differentiate the activities of two unrelated sex factors in the same donor bacterium.

Genetic evidence for subunits in the fertility repressor produced by F-like R factors

Repressor-negative mutants derived from R 100 were tested for genetic complementation. The results indicate that the active repressor is a multimer. The properties of an unusual repressor mutant are also discussed.

Serotypes of sex pili

There are situations in which direct observation of the attachment of antibody molecules may be the simplest method of detecting antigen-antibody reactions. Applied to the study of the sex pili determined by a number of transmissible bacterial plasmids, the method has distinguished four serotypes in the F-like class and two in the I-like class. Antibody was usually attached haphazardly to the pili; however, in a few cases a regular periodicity could be observed. When few antibody molecules were attached, they could frequently be individually resolved and in certain antibody-pilus combinations large antibody molecules, tentatively identified as IgM, could be seen to predominate.

Mutant Hfr strains defective in transfer: restoration by F-like and I-like de-repressed R factors

Mutants of Hfr strains ofEscherichia coliK12 defective in conjugation owing to failure to produce sex pili were isolated by resistance to F-specific phage. Tests of the ability of six de-repressed F-like R factors and four de-repressed I-like R factors to restore Hfr donor behaviour to these mutants indicated that there were several ways in which such restoration could occur. Of a total of 26 defective mutants, 12 were restored by F-like R factors. In these twelve, the function of the integrated F factor was evidently restored, because the sex pili contained F pilin subunits, distinguishable from the R pilin by serological tests. In contrast, among the four I-like R factors, only two were effective, butin all 26 defective mutants; the restored Hfr bacteria produced only I-like pili. The I-like sex factors, in restoring Hfr donor behaviour, did not therefore act by complementing the defective F.

Characteristics of E. coli K12 strains carrying both an F prime and an R factor

The interrelationship between an F prime and an F-like or I-like R factor was studied inEscherichia coliK 12 strains carrying both. The donorability of such strains suggested that these plasmids were transferred independently by their own transfer systems. Plaque formation by several male- and female-specific phage was tested; the inhibition by Flacof several female-specific phage was not affected by an F-like R factor. Neither did an R factor affect the incompatibility between Flacand Fhis. However, the surface exclusion characteristic of Flacwas lost in the presence of an F-like R factor. This is probably closely related to the simultaneous loss of donorability, the F-pilus, and the f+antigen: all may be the result of the inhibition by an R product of the formation or activity of a single F product.

Comparisons of F factors and R factors: existence of independent regulation groups in F factors

The similarity of sex pili mediated by F factors and R(fi(+)) factors and the ability of R(fi(+)) factors to control by repression the functioning of pilus genes encoded by the F factor suggested that F factors and R(fi(+)) factors are closely related. Further comparisons of the episomal properties of F factors and R(fi(+)) factors, however, indicated many differences. F factors contain information for a restriction system for phages phiII and T7. Cells containing R factors are sensitive to these phages. Furthermore, R(fi(+)) factors do not repress the F factor phiII restriction system in cells containing both an R(fi(+)) factor and an F factor. R factors and F factors are heteroimmune episomes. In addition, an R(fi(+)) factor in cells containing both an R factor and an F factor does not fully repress the expression of F-factor immunity to an incoming second F factor. R-factor and F-factor replication systems are not identical. Wild-type F-factor replication genes will complement the mutant F(ts114)lac(+) replication genes in cells containing two F factors. The F(ts114)lac(+) episome is retained when these cells are grown at 42 C; however, cells containing an R(fi(+)) factor and F(ts114)lac(+) lose the F(ts114)lac(+) when grown at 42 C, at the same rate as cells containing only the F(ts114)lac(+). The replication system of the R(fi(+)) factor will not complement the mutant F(ts114)lac(+) replication system.

Fertility regulation in F-like resistance transfer factors

Mutants of the R factor R100 have been isolated that mediate high-frequency transfer of the R factor during conjugation. Complementation tests revealed two classes of mutants, operator-constitutive and repressor-negative. Some of the latter class were suppressible by amber and ochre suppressors. The results support a simple model of regulation for the control of R-factor-mediated piliation.

Repressor-minus and operator-constitutive de-repressed mutants of F-like R factors: their effect on chromosomal transfer by HfrC

Certaindrdmutants offi+R factors, when carried by strain HfrC, allowed polarized transfer of the Hfr chromosome to occur at the normal rate. These mutants were independently shown to be repressor-sensitive and so owed their de-repression to failure to produce represser (i−). With otherdrdmutants, independently shown to be repressor-insensitive (0c), the rate of polarized chromosome transfer was as low as with the wild type and only R pili were produced by the HfrC+bacteria. These R factors, therefore, continued to produce repressor and the donor behaviour of an Hfr strain depends on functioning of the integrated F.

Chromosomal transfer mediated by de-repressed R factors in F- Escherichia coli K12

Conjugation brought about by each of eight different R factors was as efficient as with F in the production of genetic recombinants for a variety of genes located throughout the chromosome ofEscherichia coliK12.

Extrachromosomal inheritance in bacteria

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Phylogenetic relationships of drug-resistance factors and other transmissible bacterial plasmids

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