Salt Induction of Fatty Acid Elongase and Membrane Lipid Modifications in the Extreme Halotolerant Alga Dunaliella salina

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Abstract
In studies of the outstanding salt tolerance of the unicellular green alga Dunaliella salina, we isolated a cDNA for a salt-inducible mRNA encoding a protein homologous to plant β-ketoacyl-coenzyme A (CoA) synthases (Kcs). These microsomal enzymes catalyze the condensation of malonyl-CoA with acyl-CoA, the first and rate-limiting step in fatty acid elongation. Kcs activity, localized to a D. salina microsomal fraction, increased in cells transferred from 0.5 to 3.5 m NaCl, as did the level of thekcs mRNA. The function of the kcsgene product was directly demonstrated by the condensing activity exhibited by Escherichia coli cells expressing thekcs cDNA. The effect of salinity on kcsexpression in D. salina suggested the possibility that salt adaptation entailed modifications in the fatty acid composition of algal membranes. Lipid analyses indicated that microsomes, but not plasma membranes or thylakoids, from cells grown in 3.5 mNaCl contained a considerably higher ratio of C18 (mostly unsaturated) to C16 (mostly saturated) fatty acids compared with cells grown in 0.5m salt. Thus, the salt-inducible Kcs, jointly with fatty acid desaturases, may play a role in adapting intracellular membrane compartments to function in the high internal glycerol concentrations balancing the external osmotic pressure.