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Bacillus subtilis  adjusts to high osmolarity surroundings through the amassing of compatible solutes. It synthesizes the compatible solute glycine betaine from prior imported choline and scavenges many pre-formed osmostress protectants, including glycine betaine, from environmental sources. Choline is imported through the substrate-restricted ABC transporter OpuB and the closely related, but promiscuous, OpuC system, followed by its GbsAB-mediated oxidation to glycine betaine. We have investigated the impact of two MarR-type regulators, GbsR and OpcR, on  gbsAB ,  opuB , and  opuC  expression. Judging by the position of the previously identified OpcR operator in the regulatory regions of  opuB  and  opuC  [ Lee et al. (2013)  Microbiology 159, 2087−2096], and that of the GbsR operator identified in the current study, we found that the closely related GbsR and OpcR repressors use different molecular mechanisms to control transcription. OpcR functions by sterically hindering access of RNA-polymerase to the  opuB  and  opuC  promoters, while GbsR operates through a roadblock mechanism to control  gbsAB  and  opuB  transcription. Loss of GbsR or OpcR de-represses  opuB  and  opuC  transcription, respectively. With respect to the osmotic control of  opuB  and  opuC  expression, we found that this environmental cue operates independently of the OpcR and GbsR regulators. When assessed over a wide range of salinities,  opuB  and  opuC  exhibit a surprisingly different transcriptional profile. Expression of  opuB  increases monotonously in response to incrementally increase in salinity, while  opuC  transcription levels decrease after an initial up-regulation at moderate salinities. Transcription of the  gbsR  and  opcR  regulatory genes is up-regulated in response to salt stress, and is also affected through auto-regulatory processes. The  opuB  and  opuC  operons have evolved through a gene duplication event. However, evolution has shaped their mode of genetic regulation, their osmotic-stress dependent transcriptional profile, and the substrate specificity of the OpuB and OpuC ABC transporters in a distinctive fashion.

frontiers in microbiology

Two MarR-Type Repressors Balance Precursor Uptake and Glycine Betaine Synthesis in Bacillus subtilis to Provide Cytoprotection Against Sustained Osmotic Stress