SUMMARY: The moderately halophilic eubacterium has been grown at salinities over the range 5-25 % (w/v), equivalent to 0.7-3.5 M-NaCl, and the fatty acid composition determined in the late-exponential and stationary phases of batch culture. There was an increase in the proportion of cyclopropane fatty acids (CFA) as the cultures went into stationary phase at all salinities; the overall proportion of CFA was higher in the media containing more salt. The biosynthesis of CFA in was determined using radiolabelled -adenosylmethionine as the precursor incubated in cell-free extracts prepared by breaking bacteria with a French press. Compared with the activity obtained in 100 mM-phosphate buffer, the activity of CFA synthetase was inhibited by the addition of NaC1 or KC1, but stimulated up to 12-fold by added glycinebetaine, with maximum activity at 3 M. Although the specific activity of CFA synthetase in lysates from cultures grown in 0.7 or 2.1 M-NaC1 were similar in the presence of 3 M-glycinebetaine, the enzyme activity in low-salinity cultures was better adapted to function in 1 M-glycinebetaine. Shift-up experiments, in which CFA synthetase activity was assayed in cell-free extracts prepared at different times after increasing culture salinity from 0.7 to 2.1 M-NaC1, showed that the activity of the enzyme was immediately responsive to compatible solute concentration changes and indicated that enzyme induction would not be required to achieve the salt-dependent alterations in membrane lipid CFA composition A range of other compatible organic solutes stimulated CFA synthetase activity to a much lesser extent (1.8-fold) compared with glycinebetaine. It is suggested that a compatible solute, which is normally accumulated during osmo(halo)adaptation by an organism in order to contribute towards osmotic balance, does not behave passively towards intracellular proteins but can also stimulate enzyme activity.


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