σF regulon

The Alphaproteobacterium Caulobacter crescentus inhabits low-nutrient environments and can tolerate certain levels of heavy metal in these sites. It has been reported that C. crescentus responds to chromium exposure by altering expression of a large number of genes. In this work, we showed that the ECF sigma factor SigF is one of the regulatory proteins involved in the control of this transcriptional response. Microarray experiments from cells under dichromate stress showed that SigF controls a small regulon consisting of seven genes. Promoter analyses revealed that a conserved SigF-dependent sequence is located upstream of all but one gene of this regulon. Surprisingly, sigF itself was not found to be strongly auto-regulated under dichromate exposure. Furthermore, we showed that the second gene (CC3252) in the sigF operon acts as a negative regulator of SigF function, as overexpression of this putative membrane protein abolishes the transcriptional activation of sigF and its regulon under dichromate stress. Additionally, we found that substitution of one or both the conserved cysteines in the protein encoded by CC3252 (C131 and C181) affects the ability of this protein in maintaining expression of SigF-dependent genes at basal levels. Interestingly, we showed that the activation of SigF is not caused by reactive oxygen species generated as a consequence of dichromate exposure. Therefore, dichromate most probably may lead to conformational changes in the putative anti-sigma factor CC3252, releasing SigF to bind RNA polymerase core and drive transcription of its regulon. Three replicates perfomed with independent biological samples