Leucine biosynthesis in yeast : Identification of two genes (LEU4, LEU5) that affect α-Isopropylmalate synthase activity and evidence that LEU1 and LEU2 gene expression is controlled by α-Isopropylmalate and the product of a regulatory gene.

Tetrad analysis indicates that α-isopropylmalate synthase activity of yeast is determined by two separate genes, designated LEU4 and LEU5. LEU4 is identified as a structural gene. LEU5 either encodes another α-isopropylmalate synthase activity by itself or provides some function needed for the expression of a second structural gene. The properties of mutants affecting the biosynthesis of leucine and its regulation suggest that the expression of LEU1 and LEU2 (structural genes encoding isopropylmalate isomerase and β-isopropylmalate dehydrogenase, respectively) is controlled by a complex of a-isopropylmalate and a regulatory element (the LEU3 gene product). Similarities and differences between yeast and Neurospora crassa with respect to leucine biosynthesis are discussed.