CHARACTERIZATION OF A NOVEL MEMBER OF THE DEGS-DEGU REGULON AFFECTED BY SALT STRESS IN BACILLUS-SUBTILIS

As a soil bacterium also found in estuarine and marine habitats, Bacil lus subtilis has evolved various sensing and adaptation systems in ord er to face salt stress conditions. Among these regulatory mechanisms i s the DegS-DegU signal transduction system, which was previously shown to be stimulated by high salt concentrations. A search for promoters regulated in response to salt stress led to the identification of wapA , encoding a wall-associated protein, which is strongly expressed at l ow salt concentrations and almost completely repressed in the presence of 0.7 M disodium succinate. Repression of wapA transcription by salt stress was shown to require the phosphorylated form of DegU. Moreover , DegU-mediated repression of wapA occurred only in high-salt medium. Alignment between the control region of wapA and other DegU-regulated promoters allowed the identification of a putative DegU target sequenc e, AGAAN(11)TTCAG. Mutation/deletion analyses of the wapA promoter reg ion confirmed the role of the putative DegU control site in repression of wapA transcription at high salt concentrations and revealed a seco nd site of repression located downstream from the transcription start site. Since residual negative control aas observed at this second site in the absence of DegU, it seems likely that an additional repressor acts on the wapA control region to further downregulate wapA transcrip tion under salt stress conditions.