A nuclear gene required for the expression of the linear DNA-associated killer system in the yeast Kluyveromyces lactis.

The killer system of Kluyveromyces lactis is associated with two linear DNA plasmids, pGKL1 and pGKL2. The killer toxin and the immunity determinant are coded for by pGKL1. Mutations which block the expression of the killer character have been isolated. These mutations reside in a single chromosomal gene which we have named KEX1. The KEX1 gene of K. lactis has been cloned by complementation of kex1 mutations by using a recombinant plasmid pool containing the entire Kluyveromyces lactis genome, on a multicopy plasmid KEp6, which contains the Saccharomyces cerevisiae URA3 gene as a marker. Genetic analyses of strains carrying a disrupted kex1 allele demonstrated that the cloned DNA corresponded to the KEX1 gene. The cloned KEX1 gene of K. lactis has low but significant sequence homology with the KEX2 gene of Saccharomyces cerevisiae. In vivo complementation of the kex1 mutation of K. lactis by the KEX2 gene of S. cerevisiae, and complementation of the kex2 mutation of S. cerevisiae by the KEX1 gene of K. lactis, demonstrated that KEX1 of K. lactis is functionally related to the KEX2 gene of S. cerevisiae. K. lactis diploids homozygous for kex1 are deficient for sporulation.