Transient repressor effect of Fis on the growth phase-regulated osmE promoter of Escherichia coli K12

Abstract.

Transcription of the gene osmE of Escherichia coli is osmotically inducible and regulated by the growth phase. Expression of osmE is directed by a single promoter, osmE _p , which is recognized by Eσ⁷⁰ and Eσ^s, two forms of RNA polymerase using, respectively, the sigma factors σ⁷⁰ and σ^s. Eσ^s transcribes osmE _p during entry into stationary phase. Eσ⁷⁰ is responsible for osmotic induction of osmE _p during the exponential growth phase. In a search for proteins that can modulate osmE _p expression in trans, we performed electrophoretic mobility shift experiments using a DNA fragment carrying osmE _p and crude extracts from E. coli. One major retarded band was observed in these experiments. The Fis protein is responsible for this retarded band, and binds to several sites upstream and downstream of, and overlapping, the promoter region of osmE. In a fis mutant background, the kinetics of in vivo transcription of osmE _p during growth demonstrated that Fis is not responsible for the repression of the promoter seen during early exponential phase. In contrast, expression of osmE _p at elevated osmolarity during the mid-exponential growth phase is increased in the absence of Fis, demonstrating that Fis is able to act as a repressor in vivo at a particular stage of growth.