| Abstract: | Two linear killer plasmids (pGKL1 and pGKL2) from Kluyveromyces lactis stably replicated and expressed the killer phenotype in a neutral petite mutant ( rho0]) of Saccharomyces cerevisiae. However, when cytoplasmic components were introduced by cytoduction from a wild-type ( rho+]) strain of S. cerevisiae, the linear plasmids became unstable and were frequently lost from the cytoductant cells during mitosis, giving rise to nonkiller clones. The phenomenon was ascribed to the incompatibility with the introduced S. cerevisiae mitochondrial DNA (mtDNA), because the plasmid stability was restored by rho0] mutations in the cytoductant cells. Incompatibility with mtDNA was also apparent for the transmission of plasmids into diploid progeny in crosses between killer cells carrying the pGKL plasmids and rho+] nonkiller cells lacking the plasmids. High-frequency transmission of the plasmids was observed in crosses lacking mtDNA ( rho0] by rho0] crosses) and in crosses involving mutated mtDNA with large deletions of various regions of mitochondrial genome. In contrast, mutated mtDNA from various mit- mutations also exerted the incompatibility effect on the transmission of plasmids. Double-stranded RNA killer plasmids were stably maintained and transmitted in the presence of wild-type mtDNA and stably coexisted with pGKL killer plasmids in rho0] cells of S. cerevisiae. |
