Temperature-sensitive forms of large and small invertase in a mutant derived from a SUC1 strain of Saccharomyces cerevisiae.

Mutagenesis of the sucrose-fermenting (SUC1) Saccharomyces cerevisiae strain 4059-358D yielded an invertase-negative mutant (D10). Subsequent mutagenic treatment of D10 gave a sucrose-fermenting revertant (D10-ER1) that contained the same amount of large (mannoprotein) invertase as strain 4059-358D but only trace amounts of the smaller intracellular nonglycosylated enzyme. Limited genetic evidence indicated that the mutations in D10 and D10-ER1 are allelic to the SUC1 gene. The large invertases from D10-ER1 and 4059-358D were purified and compared. The two enzymes have similar specific activity and Km for sucrose, cross-react immunologically, and show the same subunit molecular weight after removal of the carbohydrate with endo-beta-N-acetylglucosaminidae H. They differ in that the large enzyme from the revertant is rapidly inactivated at 55 degrees C, whereas that from the parent is relatively stable at 65 degrees C. The small invertase in extracts of D10-ER1 is also heat sensitive as compared to the small enzyme from the original parent strain. The low level of small invertase in mutant D10-ER1 may reflect increased intracellular degradation of this heat-labile form. In several crosses of D10-ER1 with strains carrying the SUC1 or SUC3 genes, the temperature sensitivity of the large and small invertases and the low cellular level of small invertase appeared to cosegregate. These findings are evidence that SUC1 is a structural gene for invertase and that both large and small forms are encoded by a single gene. A detailed genetic analysis is presented in a companion paper.