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J. Bacteriol. doi:10.1128/JB.00775-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Salinity-dependent switching of osmolyte strategies in a moderately halophilic bacterium: glutamate induces proline biosynthesis in Halobacillus halophilus

Molecular Microbiology & Bioenergetics, Institute of Molecular Biosciences, Goethe University Frankfurt, Frankfurt am Main, Germany

^* To whom correspondence should be addressed. Email: vmueller{at}bio.uni-frankfurt.de.

	Abstract

The moderately halophilic bacterium Halobacillus halophilus copes with the salinity in its environment by the production of compatible solutes. At intermediate salinities of around 1 M NaCl cells produce glutamate and glutamine in a chloride-dependent manner (Saum, S.H., Sydow, J.F., Palm, P., Pfeiffer, F., Oesterhelt, D., and Müller, V. (2006) Biochemical and molecular characterization of the biosynthesis of glutamine and glutamate, two major compatible solutes in the moderatelly halophilic bacterium Halobacillus halophilus. J. Bacteriol. 188:6808-6815). Here, we report that H. halophilus switches its osmolyte strategy and produces proline as the dominant solute at higher salinities (2-3 M NaCl). The proline biosynthesis genes proH, proJ and proA were identified. They form a transcriptional unit and encode the pyrroline-5-carboxylate reductase, the glutamate 5-kinase, and the glutamate-5-semialdehyde dehydrogenase, respectively, catalyzing proline biosynthesis from glutamate. Expression of the genes was clearly salinity dependent and reached a maximum at 2.5 M NaCl, indicating that the pro operon is involved in salinity-induced proline biosynthesis. To address the role of anions in the process of pro gene activation and proline biosynthesis we used a cell suspension system. Chloride salts lead to the highest accumulation of proline. Interestingly, chloride could be substituted to a large extent by glutamate salts. This unexpected finding was further analyzed on the transcriptional level. The cellular mRNA levels of all three pro genes were increased up to 90fold in the presence of glutamate. A titration revealed that a minimal concentration of 0.2 M glutamate already stimulated pro gene expression. These data demonstrate that the solute glutamate is involved in the switch of osmolyte strategy from glutamate to proline as dominant compatible solute during the transition from moderate to high salinities.