Glutamate restores growth but not motility in the absence of chloride in the moderate halophile Halobacillus halophilus

Abstract

Halobacillus halophilus is a strictly chloride-dependent, moderately halophilic bacterium that synthesizes glutamate and glutamine as the major compatible solutes at intermediate NaCl concentrations. The key enzyme in production of the compatible solutes glutamine and glutamate, glutamine synthetase, is dependent on chloride on a transcriptional and activity level. This led us to ask whether exogenous supply of glutamate may relief the chloride dependence of growth of H. halophilus. Growth of H. halophilus in minimal medium at 1 M NaCl was stimulated by exogenous glutamate and transport experiments revealed a chloride-independent glutamate uptake by whole cells. Growth was largely impaired in the absence of chloride and, at the same time, glutamate and glutamine pools were reduced by 90%. Exogenous glutamate fully restored growth, and cellular glutamate and glutamine pools were refilled. Although glutamate could restore growth in the absence of chloride, another chloride-dependent process, flagellum synthesis and motility, was not restored by glutamate. The differential effect of glutamate on the two chloride-dependent processes, growth and motility, indicates that glutamate can not substitute chloride in general but apparently bypasses one function of the chloride regulon, the adjustment of pool sizes of compatible solutes.