Table of Contents:
Confronted with hyperosmotic stress, the soil bacterium Bacillus subtilis accumulates compatible solutes to maintain cell turgor. Glycine betaine is such a compatible solute that can either be taken up from the environment by various Opu-transporters or can be synthesized from the prior imported precursor choline by the dehydrogenases GbsB und GbsA. Upstream of the gbsAB gene cluster, the gbsR gene is located which encodes a MarR-type choline-responsive repressor regulating the expression of the gbsAB operon as well as the opuB operon, encoding a choline-specific ABC-Transporter (Nau-Wagner et al., 2012). This regulator mediates repression of its target genes through a road block mechanism by binding at an inverted repeat downstream of the transcriptional start site of the gbsAB und the opuB operons. A comprehensive targeted mutagenesis study based on a homology model of the dimeric GbsR protein provided evidence that choline is bound by an aromatic cage present in GbsR, which presumably matches that of substrate binding proteins acting in conjunction with ABC-Transporters. Extensive bioinformatic analyses highlighted GbsR as the prototype for a new sub-family of MarR-type regulators including members which are associated with glycine betaine sysnthesis, uptake systems for compatible solutes or oxygen reductases of the cytochrome bd-type. These types of proteins are frequently found among Archaea und Bacteria.
Genes for GbsR-type regulators associated with the closely related opuB (YvaV) und the opuC (OpcR) operon are present in B. subtilis (Nau-Wagner et al., 2012; Lee et al., 2013). Both geneclusters show a strikingly different expression pattern in response to extracellular salinities. However, they share the regulation through the GbsR-type repressor OpcR, which is involved in the re-establishment of opuC repression under high salt concentrations, a function in agreement with the salt-induced expression of the opuCR gene itself. The expression of the opuB und opuC operon is connected to the formation of biofilms throught the regulatory protein RemA, which acts as an activator of biofilm matrix synthesis, but also as an activator of the opuB und opuC transcription (Winkelman et al., 2013).
In B. subtilis no GbsR-type regulator is connected to the opuA operon. However, this is the case in the marine bacterium Bacillus infantis. OpuAR is encoded next to the opuA genecluster und acts as a choline- und glycine betaine-responsive repressor of opuA expression. In contrast to previously studied osmostress protectant uptake systems, opuA transcription in B. infantis is not enhanced in the presence of elevated osmolarities, but through the presence of its substrates.
Taken together, this work demonstrates the highly complex regulatory mechanisms leeding to the survival of bacteria under osmotically unfavourable conditins.