Expression of F plasmid traT: independence of traY----Z promoter and traJ control

TraT lipoprotein, a plasmid-specified mediator of interactions between gram-negative bacteria and their environment

The TraT protein is a cell-surface-exposed, outer membrane lipoprotein specified by large, usually conjugative, F-like plasmids. Two biological activities have been associated with the protein: (i) prevention of self-mating of cells carrying identical or closely related conjugative plasmids, by blocking the formation of stable mating aggregates; and (ii) resistance to the bactericidal activities of serum, possibly by inhibiting the correct assembly or efficient functioning of the terminal membrane attack complex of complement. The protein therefore interacts not only with components of the outer membrane but also with specific external agents. In conjugative plasmids the traT gene lies within the region necessary for the conjugal transfer of DNA (tra), although its expression is not necessarily dependent on the expression of other tra genes. Recently, however, the gene has been discovered in isolation from other tra genes in nonconjugative virulence-associated plasmids, providing further evidence that the TraT protein may have a role in pathogenesis. The nucleotide sequences of several traT genes have been determined, and comparison of the corresponding amino acid sequences suggests that a central region of five amino acid residues flanked by hydrophobic domains determines the specificity of the protein in surface exclusion. Additionally, studies of mutants with different amino acid alterations within the hydrophobic domains have shown that insertion of charged residues disrupts normal outer membrane integrity. This review considers our current knowledge of the distribution, structure, and biological role(s) of the protein. Recent applications of the protein in studies of the unusual permeability properties of the outer membrane and for the transport of foreign antigenic determinants to the bacterial cell surface are also discussed.

Transcriptional analysis of the F plasmid surface exclusion region: mapping of traS, traT, and traD transcripts

Transcription of the surface exclusion genes, traS and traT, of the F plasmid of Escherichia coli K-12 has been analyzed using S1 nuclease protection experiments. The results show that both genes possess functional promoters in vivo and that a strong transcriptional termination signal lies immediately downstream of traT. Moreover, the adjacent downstream gene, traD, is expressed from its own promoter and appears not to lie within the major transfer operon. RNA stability experiments indicate that the products of traS and traT are translated from a stable RNA message. We propose that the traS and traT promoters serve to supplement the expression of these genes from the major tra promoter, PY.

Surface exclusion genes traS and traT of the F sex factor of Escherichia coli K-12. Determination of the nucleotide sequence and promoter and terminator activities

The DNA encoding the surface exclusion genes traS and traT of the F sex factor of Escherichia coli K-12 has been sequenced and the biological activity of the various terminators and promoters determined. The data show that traS encodes a 16,861 Mr protein with no apparent signal sequence, as expected for its cytoplasmic membrane location. The protein is extremely hydrophobic. traS has its own promoter and a weak terminator region follows the gene. After the traS termination loop there is a small intergenic region before the traT promoter. The traT gene encodes a 25,932 Mr precursor for the 23,709 Mr mature protein. The amino-terminal signal peptide is 21 amino acid residues, consistent with it being an outer membrane lipoprotein. A very strong termination loop follows the gene and adjacent to this a further loop can be predicted from the sequence. These secondary structures would be expected to enhance the stability of the mRNA in the presence of 3' specific ribonucleases accounting for the apparent long half-life of the messenger. The amino acid sequence of the mature product of traT of F differs from that of R100 by only a single amino acid substitution (Gly for Ala at position 119), whereas that of pED208 (Folac) differs at 40 positions. traT lies in a region of heteroduplex homology between F and R100, and the nucleotide sequence confirms this and demonstrates that this homology breaks down immediately preceding and following the coding region. Sequence analysis shows that this is also so for pED208. Thus the entire traS of F, R100 and pED208 are very different at the DNA level. An open reading frame, preceded by a typical promoter sequence and a weak and poorly located Shine-Dalgarno sequence, follows traT and corresponds to the start of traD. Alone, this promoter appears to be inactive.

An F-derived conjugative cosmid: analysis of tra polypeptides in cosmid-infected cells

The genes involved in the conjugational transfer of F plasmid DNA are organized into three closely linked operons spanning an overall length of approximately 33 kilobase pairs of F. The entire transfer (tra) region comprising all three operons has been cloned into the cosmid vector pHC79 by in vitro recombination and packaging techniques. The transfer-proficient chimeric cosmid pRS2405 was packaged into lambda capsids, and uv-irradiated E. coli cells were infected with these DNA-filled particles. A number of polypeptides programmed by the infecting DNA were identified as tra-specified products; a traJ90 mutation on pRS2405 resulted in the significant reduction of synthesis of all detectable pRS2405-specified tra polypeptides, with the exception of TraTp.