CHS5, A GENE INVOLVED IN CHITIN SYNTHESIS AND MATING IN SACCHAROMYCES-CEREVISIAE

The CHS5 locus of Saccharomyces cerevisiae is important for wild-type levels of chitin synthase III activity. chd cells have reduced levels of this activity. To further understand the role of CHS5 in yeast, the CHS5 gene was cloned by complementation of the Calcofluor resistance phenotype of a chs5 mutant. Transformation of the mutant with a plasmi d carrying CHS5 restored Calcofluor sensitivity, wild-type cell wall c hitin levels, and chitin synthase III activity levels, DNA sequence an alysis reveals that CHS5 encodes a unique polypeptide of 671 amino aci ds with a molecular mass of 73,642 Da. The predicted sequence shows a heptapeptide repeated 10 times, a carboxy-terminal lysine-rich tail, a nd some similarity to neurofilament proteins, The effects of deletion of CHS5 indicate that it is not essential for yeast cell growth; howev er, it is important for mating, Deletion of CHS3, the presumptive stru ctural gene for chitin synthase III activity, results in a modest decr ease in mating efficiency, whereas chs5 Delta cells exhibit a much str onger mating defect, However, chd cells produce more chitin than chs3 mutants, indicating that CHS5 plays a role in other processes besides chitin synthesis, Analysis of mating mixtures of chd cells reveals tha t cells agglutinate and make contact but fail to undergo cell fusion, The chs5 mating defect can be partially rescued by FUS1 and/or FUS2, t wo genes which have been implicated previously in cell fusion, but not by FUS3, In addition, mating efficiency is much lower in fus1 fus2 x chs5 than in fus1 fus2 x wild type crosses, Our results indicate that Chs5p plays an important role in the cell fusion step of mating.