Cloning of the polyketide synthase gene atX from Aspergillus terreus and its identification as the 6-Methylsalicylic acid synthase gene by heterologous expression

The geneCAL1 (also known asCDC43) ofSaccharomyces cerevisiae encodes theβ subunit of geranylgeranyl transferase I (GGTase I), which modifies several small GTPases. Biochemical analyses of the mutant enzymes encoded bycall-1, andcdc43-2 tocdc43-7, expressed in bacteria, have shown that all of the mutant enzymes possess reduced activity, and that none shows temperature-sensitive enzymatic activities. Nonetheless, all of thecall/cdc43 mutants show temperature-sensitive growth phenotypes. Increase in soluble pools of the small GTPases was observed in the yeast mutant cells at the restrictive temperature in vivo, suggesting that the yeast prenylation pathway itself is temperature sensitive. Thecall-1 mutation, located most proximal to the C-terminus of the protein, differs from the othercdc43 mutations in several respects. An increase in soluble Rholp was observed in thecall-1 strain grown at the restrictive temperature. The temperature-sensitive phenotype ofcall-1 is most efficiently suppressed by overproduction of Rholp. Overproduction of the other essential target, Cdc42p, in contrast, is deleterious incall-1 cells, but not in othercdc43 mutants or the wild-type strains. Thecdc43-5 mutant cells accumulate Cdc42p in soluble pools andcdc43-5 is suppressed by overproduction of Cdc42p. Thus, several phenotypic differences are observed among thecall/cdc43 mutations, possibly due to alterations in substrate specificity caused by the mutations.