A SACCHAROMYCES-CEREVISIAE MUTANT DEFECTIVE IN THE KINESIN-LIKE PROTEIN KAR3 IS SENSITIVE TO NACL-STRESS

Several mutants of Saccharomyces cerevisiae showing poor growth in the presence of elevated concentrations of NaCl were isolated to identify genes involved in the osmo-stress response. One of these mutants (WAY .5-4A-11; osr11) which showed a clear 2:2 segregation of the salt-stre ss phenotype upon tetrad analysis when crossed to a wild-type strain h as been characterised. The mutation responsible for poor growth under salt-stress was recessive. The corresponding gene was cloned by comple mentation of the mutant phenotype and a 3.5-kb fragment was isolated. The sequence of this fragment matched that of KAR3, a gene previously identified to be involved in karyogamy and mitosis. Allelism of OSR11 to KAR3 was confirmed by tetrad analysis, and disruption mutants showe d the same NaCl-phenotype as the original osr11 mutation. The disrupti on mutant was more sensitive to high sucrose concentrations than the o riginal mutant was to high glucose concentrations. In a different gene tic background (W303-1A), the kar3 disruptants were less sensitive to osmo-stress than the WAY.5-4A strain. Heat-stress, nitrogen-starvation and cultivation on ethanol failed to affect the growth of osr11 and k ar3 mutants, pointing to a possible specific involvement of KAR3 in th e osmotic-stress response. Microscopic studies showed that cell divisi on of the kar3 mutants was impaired and NaCl-stress conditions aggrava ted the phenotype.