How to be moderately halophilic with broad salt tolerance: Clues from the genome of Chromohalobacter salexigens

We analyzed the amino acid composition of different categories of proteins of the moderately halophilic bacterium Chromohalobacter salexigens, as deduced from its genome sequence. Comparison with non-halophilic representatives of the γ-Proteobacteria (Escherichia coli, Pseudomonas aeruginosa, Vibrio cholerae) shows only a slight excess of acidic residues in the cytoplasmic proteins, and no significant differences were found in the acidity of membrane-bound proteins. In contrast, a very pronounced difference in mean pI value was observed for the periplasmic binding proteins of the ABC transport systems of C. salexigens and the non-halophiles E. coli and P. aeruginosa. V. cholerae, which is adapted to life in brackish water, showed intermediate values. The findings suggest that there is a major difference between the proteins of the moderate halophile C. salexigens and non-halophilic bacteria in their periplasmic proteins, exemplified by the substrate binding proteins of transport systems. The highly acidic nature of these proteins may enable them to function at high salt concentrations. The evolution of highly salt-tolerant prokaryotes may have depended on an increase in acidity of the proteins located external to the cytoplasmic membrane, enabling effective transport of nutrients into the cell.