Abstract
The moderate halophile Halobacillus halophilus is the paradigm for chloride dependent growth in prokaryotes. Recent experiments shed light on the molecular basis of the chloride dependence that is reviewed here. In the presence of moderate salinities Halobacillus halophilus mainly accumulates glutamine and glutamate to adjust turgor. The transcription of glnA2 (encoding a glutamine synthetase) as well as the glutamine synthetase activity were identified as chloride dependent steps. Halobacillus halophilus switches its osmolyte strategy and produces proline as the main compatible solute at high salinities. Furthermore, Halobacillus halophilus also shifts its osmolyte strategy at the transition from the exponential to the stationary phase where proline is exchanged by ectoine. Glutamate was found as a “second messenger” essential for proline production. This observation leads to a new model of sensing salinity by sensing the physico-chemical properties of different anions.
Highlights
Together with flourine, bromine, iodine and astatine, chlorine belongs to the seventh group in the periodic table of elements, the "halogens"
Chloride is essential for some extremely halophilic prokaryotes for osmoregulation Halophilic organisms are abundant in nature and thrive in habitats where molar concentrations of salts are present and, very low water potentials exist
A very abundant strategy to counteract the loss of water under hyperosmotic conditions is the accumulation of compatible solutes [3,4,5,6]
Summary
SFToriglauuntsercesri7ption of ect-genes is delayed after an osmotic shock compared to other genes involved in the production of compatible. One putative candidate for such a function is the regulator protein YvyD that was identified in chloride-but not in nitrate-grown Halobacillus halophilus cells [50] The latter two models have the advantage that they provide a possible mechanism, with which changes in the external salinity – that are reflected in the intracellular chloride concentrations – can be measured. The model of Halobacillus halophilus as an organism that uses an intermediate strategy for osmoregulation and physico-chemical properties of anions to regulate the processes of compatible solute accumulation opens a whole new field of regulatory mechanisms to study. Can ectoine production be triggered by other stimuli than salinity? And what happens in an ectoine-deficient mutant? The answer at least to the last question demands the availability of a genetic system that hopefully will be at hand soon
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