The letA ( ccdA ) and letD ( ccdB ) genes of the F plasmid, located just outside the sequence essential for replication, contribute to stable maintenance of the plasmid in Escherichia coli cells. The letD gene product acts to inhibit partitioning of chromosomal DNA and cell division by inhibiting DNA gyrase activity, whereas the letA gene product acts to reverse the inhibitory activity of the letD gene product. To identify the host factor(s) involved in this process, we analyzed the mutants that escaped letD expression and their suppressor, and found that the three E. coli genes tldD , tldE and zfiA participate in the process, in addition to the groE genes we reported previously. The tldD and tldE mutations made cells tolerant for letD expression, as did groES mutations, while the mutation in the zfiA gene made tldD , tldE and groES mutants LetD sensitive. We hypothesize that these gene products are factors that modulate activity of DNA gyrase along with the letD gene product; the zfiA gene product acts to inhibit interaction between the LetD protein and the A subunit of DNA gyrase, while the tldD , tldE and groE gene products act to suppress the inhibitory activity of the zfiA gene product. The tldD , tldE , and zfiA genes are located at 70.4, 96.0 and 58.2 minutes on the E. coli chromosome, respectively, and code for proteins with relative molecular masses of 51,000, 48,000 and 6800, respectively. tldD is a novel gene, but the tldE and zfiA genes proved to be the pmbA gene (production of Microcin B17) and the csrA gene (carbon storage regulator), respectively.
