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

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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 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 ::IS 10 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.

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