The miniF plasmid C protein: sequence, purification and DNA binding

Both the fipA gene of pKM101 and the pifC gene of F inhibit conjugal transfer of RP1 by an effect on traG

The mechanisms by which gene products inhibit the conjugal transfer of IncP plasmids (e.g., RP1) have been little studied. We have isolated and characterized one such gene, fipA (624 nucleotides), from the SmaI (14.8 kb)-AatII (15.6 kb) region of pKM101(IncN). This gene, which is also conserved in other IncN plasmids, is transcribed in an anticlockwise direction, probably as part of a transfer operon that includes traHI. The FipA protein (24 kDa) appears to be cytoplasmic and, when expressed from a multicopy plasmid, retards the growth of Escherichia coli WP2. The mode of action of fipA was compared with that of the apparently unrelated pifC gene from F(IncFI). Both genes inhibit the transfer of IncPalpha and IncPbeta plasmids but to different degrees. They also inhibit the mobilization of RSF1010 (which requires the RP1 pilus genes and traG) but not of CloDF13 (which encodes a traG homolog). Evidence that traG was the specific target of inhibition was obtained in an artificial system in which cloned traG was used to enhance RSF1010 mobilization via the N pilus system. Such enhancement did not occur in the presence of fipA or pifC. The availability of an in vivo assay of PifC enabled us to show that F pif operon expression increased in cells carrying F'lac and traG, but only if the traG coding sequence was intact. This finding suggested that conjugal inhibition of RP1 was most likely due to a PifC-TraG protein interaction. On phenotypic grounds inhibition of traG by fipA is also likely to occur posttranscriptionally. Whether or not the selection of traG as the inhibition target is an evolutionary tactic to limit the spread of P plasmids, we anticipate that fipA and pifC will prove useful in further investigation of the conjugal roles of traG and its homologs.

The F plasmid ccd autorepressor is a complex of CcdA and CcdB proteins

The ccd operon of plasmid F produces three proteins, CcdA, CcdB, and RepD. Prior research has established that the operon is autorepressed and that at least CcdB, but not RepD, is required for autorepression. A role for CcdA in autorepression was suggested but not clearly shown. We now present a series of biochemical experiments which show that both CcdA and CcdB proteins are required for maximal formation of protein-ccd operator complexes. We also show that CcdA and CcdB are present in a complex whether or not ccd operator is present. The clear implication is that autorepressor is a complex of CcdA and CcdB. We also map the start site of the ccd transcript thus providing the first experimental evidence for the location of the ccd promoter.

Mapping and regulation of the pifC promoter of the F plasmid

The pif region of the F plasmid, which causes abortive infection of Escherichia coli by T7 bacteriophage, is autogenously controlled by the product of the pifC gene. Here we describe the identification of the pif operon promoter by S1-nuclease mapping, and show that it is autoregulated at the transcriptional level and that its activity is modulated by integration host factor.