Two Extracytoplasmic Function Sigma Subunits, ς(E) and ς(FecI), of Escherichia coli: Promoter Selectivity and Intracellular Levels

The promoter selectivity of two extracytoplasmic function (ECF) subfamily ς subunits, ς(E) (ς(24)) and ς(FecI) (ς(18)), of Escherichia coli RNA polymerase was analyzed by using an in vitro transcription system and various promoters. The Eς(E) holoenzyme recognized only the known cognate promoters, rpoEP2, rpoHP3, and degP, and the Eς(FecI) recognized only one known cognate promoter, fecA. The strict promoter recognition properties of ς(E) and ς(FecI) are similar to those of other minor ς subunits. Transcription by Eς(E) and Eς(FecI) was enhanced by high concentrations of glutamate, as in the case of other minor ς subunits. The optimum temperature for transcription by Eς(FecI) was low, around 25°C, apparently in agreement with the high rate of iron sequestration by E. coli at low temperatures. By quantitative Western blot analysis, the intracellular levels of ς(E) and ς(FecI) in the uninduced steady-state culture of E. coli W3110 (type A) were determined to be 0.7 to 2.0 and 0.1 to 0.2 fmol per μg of total proteins (or 3 to 9 and 0.4 to 0.9 molecules per cell), respectively, and less than 1% of the level of the major ς(70) subunit.