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

Abstract

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 σ⁷⁰ family of proteins. Conserved domain 1.1 at the N-terminal end of σ⁷⁰ 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^SΔ1-50), 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^SΔ1-50. The intracellular content of σ^SΔ1-50 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 σ⁷⁰, 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.