Effects of the kar gene on cytoplasmic mixing and mitochondrial genome suppressiveness, and consequences for cytoduction of petite DNA in Saccharomyces cerevisiae

Summary

During a series of cytoduction experiments to transfer Saccharomyces cerevisiae mitochondrial genomes from one nuclear background to another, using the karl-1 nuclear fusion mutation, one of the five petite genomes used proved difficult to transfer. This genome, ϱ^- F13, was highly suppressive (90%) in its original nuclear background. Molecular and genetic studies on the putative karl-1 ϱ⁻F13 cytoductant were done to discover the nature of this difficulty. They showed that while the ϱ⁻F13 was maintained in a karl-l background, zygotes from a mating with a ϱ⁰ strain showed poor cytoplasmic mixing and therefore inefficient ϱ⁻F 13 DNA transfer into first zygotic buds. This also caused a reduction of ϱ⁻F13 suppressiveness to 20–30% in crosses with different ϱ⁺ strains. The effect was genome specific since another highly suppressive petite in the karl-l background did not show suppressiveness reduction when crossed to ϱ⁺. The nature of suppressiveness modulation is discussed. Since the ϱ⁻F13 genome was eventually transferred using a modification of the original scheme, the problems were not caused by the inability of the acceptor nuclear background to maintain the ϱ⁻F13 genome.