Regulation of the F plasmid traY promoter in Escherichia coli by host and plasmid factors.

F plasmid DNA transfer (tra) gene expression in Escherichia coli is regulated by chromosome- and F-encoded gene products. To study the relationship among these regulatory factors, we constructed low-copy plasmids containing a phi(traY'-'lacZ)hyb gene that couples beta-galactosidase and Lac permease synthesis to the F plasmid traY promoter. Wild-type transformants maintained high levels of beta-galactosidase over a broad range of culture densities. Primer extension analysis of tra mRNA from F'lac and phi(traY'-'lacZ)hyb strains indicated very similar, though not identical, transcription initiation sites. Moreover, phi(traY'-'lacZ)hyb gene expression required both TraJ and SfrA, as does tra gene expression in F+ strains. beta-Galactosidase activity was reduced approximately 30-fold in the absence of TraJ, which could be supplied in cis or in trans. In a two-plasmid system in which TraJ was supplied in trans by a lac-traJ operon fusion, phi(traY'-'lacZ)hyb expression was a linear, saturable function of traJ expression. Enzyme activity was reduced approximately tenfold in sfrA mutants. That reduction could not be attributed to an effect on the TraJ level. Several other cellular or environmental variables had only a modest effect on phi(traY'-'lacZ)hyb expression. Hyperexpression was observed at high cell density (twofold) and in anaerobic cultures (1.2- to 1.5-fold). In contrast, expression was reduced twofold in integration host factor mutants.