GLYCINEBETAINE STIMULATES, BUT NACL INHIBITS, FATTY-ACID BIOSYNTHESISIN THE MODERATELY HALOPHILIC EUBACTERIUM HX

Total fatty acid synthetase (FAS) and cyclopropane fatty acid syntheta se (CFAS) activities in cell-free lysates of the moderately-halophilic eubacterium HX, have been determined using radiolabelled malonyl-CoA and S-adenosylmethionine respectively as the precursor. The activities of FAS and CFAS were extremely low in vitro in 100 mM buffers, but we re stimulated up to 100-fold by exogenous addition of the compatible-s olute glycinebetaine to lysates; optimum activities of FAS and CFAS in vitro were obtained in 2-3 M concentrations of this compatible solute . In contrast, NaCl added to the lysate assay system was strongly inhi bitory: CFAS was 97% inhibited by 1 M NaCl whereas FAS was less sensit ive with 3 M NaCl giving 82% inhibition. When the culture medium salin ity was raised from 1 to 3 M NaCl, the endogenous activity of CFAS mea sured in vitro in lysates without additional compatible solute was app roximately doubled. This increase in CFAS activity is enough to accoun t for the known increase in CFA content which occurs when culture medi um salinity is raised, and the data are discussed in the context of th e role of intracellular compatible solutes during haloadaptation of me mbrane lipid composition.