Complex regulation of the synthesis of the compatible solute ectoine in the halophilic bacterium Chromohalobacter salexigens DSM 3043T

Abstract

The synthesis of the compatible solute ectoine, mediated by the ectABC gene products, is the main mechanism used by the halophilic bacterium Chromohalobacter salexigens to cope with osmotic stress. Evidence was found that this process is regulated at the transcriptional level. S1 protection analyses performed with RNA extracted from cells grown in minimal medium at low (0·75 M NaCl) or high (2·5 M NaCl) osmolarity suggested the existence of four promoters upstream of ectA. Two of these (PectA1 and PectA2) might be recognized by the main vegetative sigma factor σ 70, and one (PectA3) might be dependent on the general stress sigma factor σ S. The S1 protection assays suggest that PectA1 and PectA3 may be osmoregulated promoters. In addition, an internal promoter showing sequences homologous to promoters dependent on the heat-shock sigma factor σ 32 was found upstream of ectB. Transcription from PectA in C. salexigens followed a pattern typical of σ S-dependent promoters, and was reduced by 50 % in an E. coli rpoS background. These data strongly suggest the involvement of the general stress sigma factor σ S in ectABC transcription in C. salexigens. Expression of PectA–lacZ and PectB–lacZ trancriptional fusions was very high at low salinity, suggesting that ectABC may be a partially constitutive system. Both transcriptional fusions were induced during continuous growth at high temperature and their expression was reduced in cells grown in the presence of osmoprotectants (ectoine or glycine betaine) or the DNA gyrase inhibitor nalidixic acid. Moreover, PectA–lacZ expression was negatively modulated in cells grown with an excess of iron (FeCl3). Measurement of ectoine levels in the presence of glycine betaine at different NaCl concentrations suggests that an additional post-transcriptional control may occur as well.