Identification by functional analysis of the gene encoding alpha-isopropylmalate synthase II (LEU9) in Saccharomyces cerevisiae.

The function of the open reading frame (ORF) YOR108w of Saccharomyces cerevisiae has been analysed. The deletion of this ORF from chromosome XV did not give an identifiable phenotype. A mutant in which both ORF YOR108w and LEU4 gene have been deleted proved to be leucine auxotrophic and alpha-isopropylmalate synthase (alpha-IPMS)-negative. This mutant recovered alpha-IPMS activity and a Leu(+) phenotype when transformed with a plasmid copy of YOR108w. These data and the sequence homology indicated that YOR108w is the structural gene for alpha-IPMS II, responsible for the residual alpha-IPMS activity found in a leu4Delta strain. The leu4Delta strain appeared to be very sensitive to the leucine analogue trifluoroleucine. In the absence of leucine, its growth was not much impaired in glucose but more on non-fermentable carbon sources.