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 mitochondrial DNA-deficient (rho 0) MATa or MAT alpha haploids of Saccharomyces cerevisiae, but were unstable and frequently lost in rho 0 MATa/MAT alpha diploids, suggesting that the replication of pGKL plasmids was under the control of the MAT locus. In MATa/MAT alpha cells of S. cerevisiae, the MAT alpha gene product (alpha 2) is combined with the MATa gene product (a1) and the resultant protein, a1-alpha 2, acts to repress the expression of haploid specific genes. Experiments showed that the K. lactis linear plasmids were stably maintained in rho 0 mata1/MAT alpha diploids, indicating that the a1-alpha 2 repressor interfered with the stability of pGKL2. It was revealed by computer analysis that the consensus sequence homologous to the a1-alpha 2 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.