Abstract
Halobacillus halophilus, a moderately halophilic bacterium isolated from salt marshes, produces various compatible solutes to cope with osmotic stress. Glutamate and glutamine are dominant compatible solutes at mild salinities. Glutamine synthetase activity in cell suspensions of Halobacillus halophilus wild type was shown to be salt dependent and chloride modulated. A possible candidate to catalyze glutamine synthesis is glutamine synthetase A2, whose transcription is stimulated by chloride. To address the role of GlnA2 in the biosynthesis of the osmolytes glutamate and glutamine, a deletion mutant (ΔglnA2) was generated and characterized in detail. We compared the pool of compatible solutes and performed transcriptional analyses of the principal genes controlling the solute production in the wild type strain and the deletion mutant. These measurements did not confirm the hypothesized role of GlnA2 in the osmolyte production. Most likely the presence of another, yet to be identified enzyme has the main contribution in the measured activity in crude extracts and probably determines the total chloride-modulated profile. The role of GlnA2 remains to be elucidated.
Highlights
Halophilic bacteria are truly fascinating microorganisms that can grow over a wide range of salinities with identical growth rates demonstrating a high flexibility in coping with salt stress
The threefold increase of the GlnA2 observed by increasing the salinity from 0.4 to 3 M NaCl is in good agreement with the fourfold increase in expression of glnA2 gene determined previously (Saum et al, 2006) and supports the hypothesis that GlnA2 is the essential glutamine synthetase in solute production
This genetic organization is typical for Bacillus sp. and the derived amino acids sequence of glnA1 is 81% identical to that of the corresponding protein from Bacillus subtilis (Saum et al, 2006), which is a classical enzyme involved in nitrogen metabolism
Summary
Halophilic bacteria are truly fascinating microorganisms that can grow over a wide range of salinities (from 0.5 to 3.0 M NaCl) with identical growth rates demonstrating a high flexibility in coping with salt stress. The most prominent challenge a moderate halophile faces in its habitat is the loss of water from the cytoplasm at high salinities (Ventosa et al, 1998; Oren, 2008). This is combated by the accumulation of compatible solutes, small molecules that do not interfere with the primary metabolism (Galinski, 1995; Kempf and Bremer, 1998; Roeßler and Müller, 2001b; Roberts, 2005). Various studies (Dohrmann and Müller, 1999; Roeßler and Müller, 2001a, 2002; Sewald et al, 2007; Köcher et al, 2009) unraveled a chloride modulon that mediates sensing of the external salt concentration and transmission of information to enzymes whose activities are modulated by chloride or to genes whose transcription is regulated by the anion (Saum and Müller, 2008b)
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