Transcriptional analysis of the acid-inducible asr gene in enterobacteria.

We show here that transcription of the asr gene in Escherichia coli, Salmonella enterica serovar Typhimurium, Klebsiella pneumoniae and Enterobacter cloacae is strongly dependent on the acidification level of the growth medium, with maximal induction at pH 4.0-4.5 as determined by Northern hybridization analysis. Previous gene array analyses have also shown that asr is the most acid-induced gene in the E. coli genome. Sequence alignment of the asr promoters from different enterobacterial species identified a highly conserved region located at position -70 to -30 relative to the asr transcriptional start site. By deletion of various segments of this region in the E. coli asr promoter it was shown that sequences upstream from the -40 position were important for induction. Transcription from the E. coli asr promoter was demonstrated to be growth-phase-dependent and to require the alternative sigma factor RpoS (sigma(S)) in stationary phase. Transcription of the asr gene was also found to be subject to negative control by the nucleoid protein H-NS.