Yeast LEU5 is a PET-like gene that is not essential for leucine biosynthesis

Alpha-IPM synthase catalyzes the first committed step in leucine biosynthesis in the yeast S. cerevisiae. LEU4 is known to encode this enzyme activity. A second gene, LEU5, has been proposed to encode a second enzyme with this activity. We cloned LEU5 and genetically defined the locus. LEU5 maps to chromosome VIII and is tightly linked to CEN8. Five different mutations in LEU5 were analyzed: a site-directed deletion and a disruption, as well as three distinct mutations produced by chemical mutagenesis. In a leu4 background, each leu5 mutation causes a Leu--phenotype; in a LEU4 background, none of the mutations alters the Leu+ phenotype. This shows that LEU5 is not essential for leucine biosynthesis. In either a leu4 or LEU4 background, each leu5 mutation causes a glycerol--phenotype. This operationally defines LEU5 as a PET gene. Two distinct suppressors of the Pet--phenotype of leu5 strains have been isolated. These suppressors revert the Pet--phenotype of each of four mutant leu5 alleles that were tested. Suppression occurs regardless of the allele at LEU4. Moreover, the suppressors co-revert the Leu--phenotype for each of the four leu5 mutations that is combined with a leu4 allele. This establishes the presence of a gene other than LEU5 that encodes a second alpha-IPM synthase. Further analysis provided no evidence for synthase activity that is encoded by LEU5.