The ECF sigma factor σT is involved in osmotic and oxidative stress responses in Caulobacter crescentus

Abstract

Sigma factors of the ECF subfamily are important regulators of stress responses in bacteria. Analysis of Caulobacter crescentus genome sequence has indicated the presence of 13 members of the ECF (extracytoplasmic function) subfamily, suggesting that these regulators play an important role in C. crescentus physiology. This work describes the characterization of two highly similar C. crescentus ECF sigma factors, σU and σT. The corresponding genes are not essential under normal growth conditions and absence of σU does not impair bacterial resistance to the environmental stresses tested. However, absence of σT significantly affects the ability of C. crescentus cells to survive osmotic and oxidative stress. Using transcription fusions to sigT and sigU upstream regions we demonstrate that both genes are induced by osmotic stress in a σT-dependent manner. Determination of sigU and sigT transcription start sites revealed an identical promoter motif, typical of ECF-dependent promoters. Transcriptome analysis revealed 40 putative members of the σT regulon, including sigU and sigR, encoding another ECF subfamily member, and genes involved in general stress responses and cell envelope functions. Twenty of those genes exhibit the sigT/sigU promoter motif in their upstream regions. Our data indicate a role of σT in distinct stress responses in C. crescentus. © 2007 The Authors.