Mating type locus‐dependent stability of the Kluyveromyces linear pGKL plasmids in Saccharomyces cerevisiae

The linear killer plasmids, pGKL1 and pGKL2, from Kluyveromyces lactis stably replicated in mitochondria1 DNA‐deficient (ρ⁰) MATa or MATα haploids of Saccharomyces cerevisiae, but were unstable and frequently lost in ρ° MATa/MATα diploids, suggesting that the replication of pGKL plasmids was under the control of the MAT locus. In MATa/MATα cells of S. cerevisiae, the MATα gene product (α2) is combined with the MATa gene product (a1) and the resultant protein, a1‐α2, acts to repress the expression of haploid specific genes. Experiments showed that the K. lactis linear plasmids were stably maintained in ρ⁰ mata1/MATα diploids, indicating that the a1‐α2 repressor interfered with the stability of pGKL2. It was revealed by computer analysis that the consensus sequence homologous to the a1‐α2 repressor binding site occurred within the coding regions of pGKL2 genes which were presumed to be essential for the plasmid replication. Since the plasmids were stably maintained in diploids of K. lactis, the mating type control must not be working there.