An N-Terminally Truncated RpoS (σ(S)) Protein in Escherichia coli Is Active In Vivo and Exhibits Normal Environmental Regulation Even in the Absence of rpoS Transcriptional and Translational Control Signals

RpoS (σ(S)) in Escherichia coli is a stationary-phase-specific primary sigma factor of RNA polymerase which is 330 amino acids long and belongs to the eubacterial σ(70) family of proteins. Conserved domain 1.1 at the N-terminal end of σ(70) has been shown to be essential for RNA polymerase function, and its deletion has been shown to result in a dominant-lethal phenotype. We now report that a σ(S) variant with a deletion of its N-terminal 50 amino acids (σ(S)Δ1-50), when expressed in vivo either from a chromosomal rpoS::IS10 allele (in rho mutant strains) or from a plasmid-borne arabinose-inducible promoter, is as proficient as the wild type in directing transcription from the proU P1 promoter; at three other σ(S)-dependent promoters that were tested (osmY, katE, and csiD), the truncated protein exhibited a three- to sevenfold reduced range of activities. Catabolite repression at the csiD promoter (which requires both σ(S) and cyclic AMP [cAMP]-cAMP receptor protein for its activity) was also preserved in the strain expressing σ(S)Δ1-50. The intracellular content of σ(S)Δ1-50 was regulated by culture variables such as growth phase, osmolarity, and temperature in the same manner as that described earlier for σ(S), even when the truncated protein was expressed from a template that possessed neither the transcriptional nor the translational control elements of wild-type rpoS. Our results indicate that, unlike that in σ(70), the N-terminal domain in σ(S) may not be essential for the protein to function as a sigma factor in vivo. Furthermore, our results suggest that the induction of σ(S)-specific promoters in stationary phase and during growth under conditions of high osmolarity or low temperature is mediated primarily through the regulation of σ(S) protein degradation.