Patterns of sexual recombination in enteric bacteria

Comparative Genomics of the Conjugation Region of F-like Plasmids: Five Shades of F

The F plasmid is the foremost representative of a large group of conjugative plasmids, prevalent in Escherichia coli, and widely distributed among the Enterobacteriaceae. These plasmids are of clinical relevance, given their frequent association with virulence determinants, colicins, and antibiotic resistance genes. Originally defined by their sensitivity to certain male-specific phages, IncF plasmids share a conserved conjugative system and regulatory circuits. In order to determine whether the genetic architecture and regulation circuits are preserved among these plasmids, we analyzed the natural diversity of F-like plasmids. Using the relaxase as a phylogenetic marker, we identified 256 plasmids belonging to the IncF/ MOB_F12group, present as complete DNA sequences in the NCBI database. By comparative genomics, we identified five major groups of F-like plasmids. Each shows a particular operon structure and alternate regulatory systems. Results show that the IncF/MOB_F12 conjugation gene cluster conforms a diverse and ancient group, which evolved alternative regulatory schemes in its adaptation to different environments and bacterial hosts.

Recombination Events in the Bacterial Genus Nocardia

Genetic recombination was demonstrated in the bacterial genus Nocardia. The compatibility mechanism governing recombination differs from that of Escherichia coli and the streptomycetes. Mutants of N. erythropolis or N. canicruria, of homologous origin, were incompatible, but mutants of heterologous origin were compatible. Such a compatibility system is suggestive of classic heterothallism which up to now has not been known among the bacteria.

Genetic Characterization of a Stable F' lac Plasmid

A mutant F' plasmid has been isolated in a strain of Salmonella typhimurium harboring F(ts114)lac. This mutant, designated FlacS, exhibits unique genetic stability in strains of S. typhimurium and Escherichia coli. It shows no thermolability and is lost at frequencies of 20 to 100 times less than the wild-type F'lac (F42) in the same genetic backgrounds. The FlacS is also insensitive to conventional plasmid curing agents, whereas both F(ts114)lac and F42 are readily cured. The nature of the mutation(s) conferring stability to the FlacS is unclear, but plasmid linkage has been established. The high frequency of conjugal transfer of the FlacS and its behavior in recombination-deficient strains of S. typhimurium and E. coli argue against its stability being due to stable chromosomal integration. The FlacS is also capable of transferring chromosomal markers in S. typhimurium and E. coli mating systems. No major differences in chromosomal mobilization have been observed among F42, F(ts114)lac, and FlacS donors of either genus.

Synthesis of FinP RNA by plasmids F and pSLT is regulated by DNA adenine methylation

DNA adenine methylase mutants of Salmonella typhimurium contain reduced amounts of FinP, an antisense RNA encoded by the virulence plasmid pSLT. Lowered FinP levels are detected in both Dam- FinO+ and Dam- FinO- backgrounds, suggesting that Dam methylation regulates FinP production rather than FinP half-life. Reduced amounts of F-encoded FinP RNA are likewise found in Dam- mutants of Escherichia coli. A consequence of FinP RNA scarcity in the absence of DNA adenine methylation is that Dam- mutants of both S. typhimurium and E. coli show elevated levels of F plasmid transfer. Inhibition of F fertility by the S. typhimurium virulence plasmid is also impaired in a Dam- background.

TRANSFER OF EPISOMIC ELEMENTS TO PROTEUS. I. TRANSFER OF F-LINKED CHROMOSOMAL DETERMINANTS

Falkow, Stanley (Walter Reed Army Institute of Research, Washington, D.C.), J. A. Wohlhieter, R. V. Citarella, and L. S. Baron. Transfer of episomic elements to Proteus. I. Transfer of F-linked chromosomal determinants. J. Bacteriol. 87:209-219. 1964.-F-linked lac(+) genes may be transferred from Escherichia coli to several species of Proteus by conjugation. Usually the transferred genetic elements are markedly unstable in Proteus, but repeated plating permits the selection of relatively stable Proteus lac(+) strains. Proteus strains carrying F-linked lac(+) markers are heterogenotes and limited donors for lac(+). In addition, both the fertility and lac(+) property may be eliminated from Proteus by treatment with acridine orange. Escherichia and Proteus possess very different overall deoxyribonucleic acid (DNA) base compositions. In CsCl density gradients of DNA extracted from Proteus lac(+) strains, the acquisition of Escherichia genes by Proteus may be correlated with the addition of a physically recognizable high molecular weight, native DNA fraction of Escherichia base composition. Proteus lac(+) strains synthesize a beta-galactosidase which is indistinguishable from E. coli enzyme by several criteria. Despite this specificity, the regulatory functions of Escherichia lac(+) genes appear to be impaired in Proteus.

Derepression of F-lac in Salmonella typhimurium by a determinant for kanamycin resistance

When a determinant for neomycin-kanamycin resistance (K) was transferred by an F-lacfactor intoSalmonella typhimurium, the resulting KF-lacstrain was sensitive to the male-specific phage μ2and F-lacwas derepressed. However, F-lacalone is repressed inS. typhimurium. When kanamycin resistance is spontaneously lost fromS. typhimuriumKF-lacan element persists which derepresses F-lacinS. typhimurium. The results are consistent with the hypothesis that a locusder, for derepression of F-lacinS. typhimurium, lies on the K plasmid. The R factor Rldrd19 is derepressed in K12 but is repressed inS. typhimurium. It also is derepressed byder. In contrast to F-lacand R1, another R factor, R136drdH8, is derepressed in both K12 andS. typhimurium, so that the intervention ofderis unnecessary for its derepression in the salmonella host.

Drug resistance of enteric bacteria. VI. Introduction of bacteriophage P1CM into Salmonella typhi and formation of PldCM and F-CM elements

Kondo, Eiko (Gunma University, Maebashi, Japan), and Susumu Mitsuhashi. Drug resistance of enteric bacteria. VI. Introduction of bacteriophage P1CM into Salmonella typhi and formation of P1dCM and F-CM elements. J. Bacteriol. 91:1787-1794. 1966.-Bacteriophage P1CM was introduced into Salmonella typhi by means of both phage infection and conjugation with Escherichia coli F(+) lysogenic for the phage. Upon incubation with a P1CM phage lysate, S. typhi and S. abony yield CM(r) cells which are lysogenic for P1CM, but S. typhimurium LT2 does not. The P1CM phage is adsorbed slightly to S. typhi, but no infectious centers are formed when the phage is plated on this strain. Tests on P1CM-adsorbing capacity of the S. typhi P1CM(+) strain and on plaque formation and transduction ability of the recovered phage from this strain indicated that the cell and the phage population did not have any special advantage over the original cell and phage population. Conjugation of S. typhi with E. coli F(+) carrying P1CM(+) gave three types of S. typhi CM(r) clones: those which carry the whole P1CM phage, those with the P1dCM element, and those with nontransferable CM(r). The second type has the F factor and is sensitive to f phages in spite of its typical behavior, serologically and biochemically, as S. typhi. It can donate the P1dCM and F(+) characters to E. coli F(-) or F(-)/P1 strains. As a consequence of conjugation with the E. coli F(+) strain, the CM(r) character of the third type of S. typhi, the nontransferable CM(r) element, acquired conjugational transferability, owing to the formation of the element, F-CM. This element can be transferred to an E. coli F(-) strain at a very high frequency (ca. 10(0)). Both the F and CM(r) determinants are jointly transduced with P1 phage and are jointly eliminated by acridine dye treatment.