kem Mutations affect nuclear fusion in Saccharomyces cerevisiae

Abstract

We have identified mutations in three genes of Saccharomyces cerevisiae, KEM1, KEM2 and KEM3, that enhance the nuclear fusion defect of kar 1-1 yeast during conjugation. The KEM1 and KEM3 genes are located on the left arm of chromosome VII. Kem mutations reduce nuclear fusion whether the kem and the kar 1-1 mutations are in the same or in different parents (i.e., in both kem kar 1-1 x wild-type and kem x kar 1-1 crosses). kem 1 x kem 1 crosses show a defect in nuclear fusion, but kem 1 x wild-type crosses do not. Mutant kem 1 strains are hypersensitive to benomyl, lose chromosomes at a rate 10-20-fold higher than KEM+ strains, and lose viability upon nitrogen starvation. In addition, kem 1/kem 1 diploids are unable to sporulate. Cells containing a kem 1 null allele grow very poorly, have an elongated rod-shape and are defective in spindle pole body duplication and/or separation. The KEM 1 gene, which is expressed as 5.5-kb mRNA transcript, contains a 4.6-kb open reading frame encoding a 175-kD protein.