Therefore, both σF-dependent genes with a putative assigned function appear to play a role in sulfate acquisition by cells. Interestingly, Hu et al. (2005) found a strong down-regulation of a Caulobacter sulfate ABC transport system under chromate and dichromate exposure. While this detoxification

strategy PD-1/PD-L1 signaling pathway apparently contributes to decrease the concentration of chromate and dichromate in the cells [4], sulfate uptake from the extracellular environment might be significantly affected. Alternative sources such as degradation of sulfur-containing amino-acids [25] and organosulfonate metabolism [26] can be used to counteract this sulfur uptake limitation [1, 27–29]. It is therefore conceivable that induction of CC2748 and CC3257 could supply cells with sulfate. This is consistent with the observation that in Arthrobacter sp. strain FB24 and Pseudomonas putida, LY2835219 mouse chromate exposure also results in increased levels of proteins potentially involved AZD8186 molecular weight in reversing the effects of cellular sulfur limitation, such as transporters of alternative sulfur sources [27, 28]. Curiously, none of the most representative functional categories up-regulated under chromate, dichromate or cadmium exposure (protection against oxidative stress and reduction of intracellular

metal concentration) were found to be controlled by σF, indicating that additional molecular systems are engaged in C. crescentus response to these metals. In fact,

we previously reported the involvement of the paralogous sigma factors σT and σU in the control of response to chromium and PLEK2 cadmium [14, 15, 30] and σE in response to cadmium [14, 15, 30]. The observation that σF, σE and σT/σU regulate distinct sets of genes indicates that each of these sigma factors make a different contribution to the C. crescentus response to metal stress. Together, σF, σE, σT and σU are responsible for the induction of 20% of the genes previously found to be up-regulated under cadmium stress and σF, σT and σU control the expression of about 12% of genes induced following Caulobacter exposure to chromate or dichromate (Additional file 1: Table S1). Therefore, transcriptional regulators other than σF, σE, σT and σU appear to be involved in the response to chromate, dichromate and cadmium. The existence of several molecular systems contributing to the transcriptional response to metal stresses could explain why the absence of sigF, CC2906 or CC3255 does not decrease the viability of Caulobacter cells under dichromate or cadmium stresses. In agreement, we previously reported that σE elicits a rapid response to cadmium, but cells lacking rpoE are not impaired in survival to this stress condition [14, 15, 30]. Interestingly, sigF is not highly induced under either chromium or cadmium stress, different from what was observed for other ECF sigma factor genes such as rpoE and sigT in C.