USE OF SYNTHETIC LETHAL MUTANTS TO CLONE AND CHARACTERIZE A NOVEL CTPSYNTHETASE GENE IN SACCHAROMYCES-CEREVISIAE

In the pyrimidine biosynthetic pathway, CTP synthetase catalyses the c onversion of uridine 5'-triphosphate (UTP) to cytidine 5'-triphosphate (CTP). In the yeast Saccharomyces cerevisiae, the URA7 gene encoding this enzyme was previously shown to be nonessential for cell viability . The present paper describes the selection of synthetic lethal mutant s in the CTP biosynthetic pathway that led us to clone a second gene, named URA8, which also encodes a CTP synthetase. Comparison of the pre dicted amino acid sequences of the products of URA7 and URA8 shows 78% identity. Deletion of the URA8 gene is viable in a haploid strain but simultaneous presence of null alleles both URA7 and URA8 is lethal. B ased on the codon bias values for the two genes and the intracellular concentrations of CTP in strains deleted for one of the two genes, rel ative to the wild-type level, URA7 appears to be the major gene for CT P biosynthesis. Nevertheless, URA8 alone also allows yeast growth, at least under standard laboratory conditions.