Abstract. The isolation and biochemical characterization of a Saccharomyces cerevisiae mutant, which grows only when emulsified myristic, palmitic, stearic, or oleic acid is added to the growth medium, is described. The mutant contains an enzymatically inactive fatty acid synthetase complex. The gene affected, preliminarily designated by the symbol fas, exhibits a 2:2 nuclear inheritance pattern. The formation of fatty acid synthetase in yeast is constitutive and not subject to repression by long chain fatty acids. After extensive purification, the mutant fatty acid synthetase was obtained as an essentially homogeneous protein with a sedimentation constant identical to that of the wild type enzyme. A systematic study of the seven reaction steps involved in fatty acid biosynthesis revealed that the enzyme catalyzing the condensation of acetate and malonate to acetoacetate was completely inactive in the mutant. The other six coinponent enzymes had identical specific activities in the mutant and in the wild type fatty acid synthetase complexes. It is concluded that the mutant described harbors a missense mutation in the structural genes of either the condensing enzyme of the "acyl carrier protein" component of the fatty acid synthetase complex.

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