Evidence for transcription attenuation rendering cryptic a sigmaS-dependent promoter of the osmotically regulated proU operon of Salmonella typhimurium

In vitro properties of RpoS (sigma(S)) mutants of Escherichia coli with postulated N-terminal subregion 1.1 or C-terminal region 4 deleted

Derivatives of the stationary-phase sigma factor sigma(S) of Escherichia coli lacking either of two conserved domains, the postulated N-terminal subregion 1.1 or the C-terminal region 4, were shown to be competent in vitro for transcription initiation from several sigma(S)-dependent promoters on supercoiled DNA templates. Unlike wild-type sigma(S), however, the deletion derivatives were inactive on relaxed templates. The anomalous slow electrophoretic mobility of sigma(S) on denaturing gels was corrected by deletion of subregion 1.1, suggesting that this domain in sigma(S) may be structurally and functionally analogous to subregion 1.1 of sigma(70), substitutions in which have previously been shown to rectify the anomalous electrophoretic migration of sigma(70) (V. Gopal and D. Chatterji, Eur. J. Biochem. 244:614-618, 1997).

An N-terminally truncated RpoS (sigma(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 (sigma(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 sigma(70) family of proteins. Conserved domain 1.1 at the N-terminal end of sigma(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 sigma(S) variant with a deletion of its N-terminal 50 amino acids (sigma(S)Delta1-50), 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 sigma(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 sigma(S) and cyclic AMP [cAMP]-cAMP receptor protein for its activity) was also preserved in the strain expressing sigma(S)Delta1-50. The intracellular content of sigma(S)Delta1-50 was regulated by culture variables such as growth phase, osmolarity, and temperature in the same manner as that described earlier for sigma(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 sigma(70), the N-terminal domain in sigma(S) may not be essential for the protein to function as a sigma factor in vivo. Furthermore, our results suggest that the induction of sigma(S)-specific promoters in stationary phase and during growth under conditions of high osmolarity or low temperature is mediated primarily through the regulation of sigma(S) protein degradation.

In vivo expression from the RpoS-dependent P1 promoter of the osmotically regulated proU operon in Escherichia coli and Salmonella enterica serovar Typhimurium: activation by rho and hns mutations and by cold stress

Unlike the sigma(70)-controlled P2 promoter for the osmotically regulated proU operon of Escherichia coli and Salmonella enterica serovar Typhimurium, the sigma(s)-controlled P1 promoter situated further upstream appears not to contribute to expression of the proU structural genes under ordinary growth conditions. For S. enterica proU P1, there is evidence that promoter crypticity is the result of a transcription attenuation phenomenon which is relieved by the deletion of a 22-base C-rich segment in the transcript. In this study, we have sought to identify growth conditions and trans-acting mutations which activate in vivo expression from proU P1. The cryptic S. enterica proU P1 promoter was activated, individually and additively, in a rho mutant (which is defective in the transcription termination factor Rho) as well as by growth at 10 degrees C. The E. coli proU P1 promoter was also cryptic in constructs that carried 1.2 kb of downstream proU sequence, and in these cases activation of in vivo expression was achieved either by a rho mutation during growth at 10 degrees C or by an hns null mutation (affecting the nucleoid protein H-NS) at 30 degrees C. The rho mutation had no effect at either 10 or 30 degrees C on in vivo expression from two other sigma(s)-controlled promoters tested, those for osmY and csiD. In cells lacking the RNA-binding regulator protein Hfq, induction of E. coli proU P1 at 10 degrees C and by hns mutation at 30 degrees C was still observed, although the hfq mutation was associated with a reduction in the absolute levels of P1 expression. Our results suggest that expression from proU P1 is modulated both by nucleoid structure and by Rho-mediated transcription attenuation and that this promoter may be physiologically important for proU operon expression during low-temperature growth.

Superimposition of tyrR protein-mediated regulation on osmoresponsive transcription of Escherichia coli proU in vivo

Osmotic regulation of proU expression in the enterobacteria is achieved, at least in part, by a repression mechanism involving the histone-like nucleoid protein H-NS. By the creation of binding sites for the TyrR regulator protein in the vicinity of the sigma70-controlled promoter of proU in Escherichia coli, we were able to demonstrate a superposed TyrR-mediated activation by L-phenylalanine (Phe), as well as repression by L-tyrosine, of proU expression in vivo. Based on the facts that pronounced activation in the presence of Phe was observed even at a low osmolarity and that the affinity of binding of TyrR to its cognate sites on DNA is not affected by Phe, we argue that H-NS-mediated repression of proU at a low osmolarity may not involve a classical silencing mechanism. Our data also suggest the involvement of recruited RNA polymerase in the mechanism of antirepression in E. coli.