Isolation and characterization of Escherichia coli hag operator mutants whose hag48 expression has become repressible by a Salmonella H1 repressor

The expression of an Escherichia coli K12 flagellin gene, hagA48, is insensitive to the Salmonella H1 repressor (rh1^-).By selecting merodiploid cells H2-rh1^on-off/F′hag48 for motility in the presence of anti-H48 serum, mutants which had escaped from inhibition by the serum because of repression of their hag48 expression by rh1⁺ were isolated. Their nucleotide sequences were examined in the region containing the promoter, the position of which was confirmed by S1 nuclease analysis of the transcriptional initiation site. The two independently isolated mutants had the same heptamer insertion AGACGAT at a site overlapping with the promoter sequence, creating a putative operator sequence homologous to Salmonella H1, but not to H2. Other candidates for operator mutants had reduced flagellar synthesis because of mutations between the transcriptional and translational initiation sites or in the structural gene. The sequence analysis also revealed a repetitive extragenic palindrome (REP) consensus sequence and a transcriptional terminator of hag48 in a small, functionally unknown open reading frame (ORF).