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, ^- F13, was highly suppressive (90%) in its original nuclear background. Molecular and genetic studies on the putative karl-1 ^–F13 cytoductant were done to discover the nature of this difficulty. They showed that while the ^–F13 was maintained in a karl-l background, zygotes from a mating with a ⁰ strain showed poor cytoplasmic mixing and therefore inefficient ^–F 13 DNA transfer into first zygotic buds. This also caused a reduction of ^–F13 suppressiveness to 20–30% in crosses with different ⁺ strains. The effect was genome specific since another highly suppressive petite in the karl-l background did not show suppressiveness reduction when crossed to ⁺. The nature of suppressiveness modulation is discussed. Since the ^–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 ^–F13 genome.