AUTHOR=Warmbold Bianca , Ronzheimer Stefanie , Freibert Sven-Andreas , Seubert Andreas , Hoffmann Tamara , Bremer Erhard 

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

JOURNAL=Frontiers in Microbiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2020.01700

DOI=10.3389/fmicb.2020.01700

ISSN=1664-302X

ABSTRACT=<p><italic>Bacillus subtilis</italic> 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 <italic>gbsAB</italic>, <italic>opuB</italic>, and <italic>opuC</italic> expression. Judging by the position of the previously identified OpcR operator in the regulatory regions of <italic>opuB</italic> and <italic>opuC</italic> [<xref ref-type="bibr" rid="B45">Lee et al. (2013)</xref> 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 <italic>opuB</italic> and <italic>opuC</italic> promoters, while GbsR operates through a roadblock mechanism to control <italic>gbsAB</italic> and <italic>opuB</italic> transcription. Loss of GbsR or OpcR de-represses <italic>opuB</italic> and <italic>opuC</italic> transcription, respectively. With respect to the osmotic control of <italic>opuB</italic> and <italic>opuC</italic> expression, we found that this environmental cue operates independently of the OpcR and GbsR regulators. When assessed over a wide range of salinities, <italic>opuB</italic> and <italic>opuC</italic> exhibit a surprisingly different transcriptional profile. Expression of <italic>opuB</italic> increases monotonously in response to incrementally increase in salinity, while <italic>opuC</italic> transcription levels decrease after an initial up-regulation at moderate salinities. Transcription of the <italic>gbsR</italic> and <italic>opcR</italic> regulatory genes is up-regulated in response to salt stress, and is also affected through auto-regulatory processes. The <italic>opuB</italic> and <italic>opuC</italic> 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.</p>