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-stress phenotype upon tetrad analysis when crossed to a wild-type strain has been characterised. The mutation responsible for poor growth under salt-stress was recessive. The corresponding gene was cloned by complementation 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 showed the same NaCl-phenotype as the original osr11 mutation. The disruption mutant was more sensitive to high sucrose concentrations than the original mutant was to high glucose concentrations. In a different genetic 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 kar3 mutants, pointing to a possible specific involvement of KAR3 in the osmotic-stress response. Microscopic studies showed that cell division of the kar3 mutants was impaired and NaCl-stress conditions aggravated the phenotype.