Thirty thermotolerant mutants of a haploid strain of Saccharomyces cerevisiae have been isolated using chemical and physical methods of mutagenesis and genetically characterised. These could survive a 72-h incubation at 44 degrees C. Some of them produced relatively higher levels of ethanol at 37 degrees C, 40 degrees C and 44 degrees C when compared with their parent strain. Like the parent, the mutant strains followed a characteristic thermal inactivation pattern at 44 degrees C except that the process was significantly delayed. Microscopic examination of the mutant cells in YEPD broth incubated at 44 degrees C revealed an enlargement of cell size by three or four times. Their genetic characterisation showed that crosses involving each of the mutants and the thermosensitive wild type produced 1:3 thermotolerant and thermosensitive phenotypes instead of the 1:1 ratio usually expected. The allelism test, where each thermotolerant mutant was crossed with another, produced thermosensitive recombinants to the level of 25%. The haploid cells of mutants were found to revert quite frequently but diploids constructed thereof were found to be perfectly stable. We therefore speculate that various mutations are in one and the same gene rather than being genetically different, as due to phenotypic similarity revertants could be easily mistaken for recombinants. This gene controlling thermotolerance was named tet11 and is recessive in nature.
|Journal||New Genetics and Society|
|Publication status||Published (in print/issue) - Mar 1996|