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 ϱ(0) 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.

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Sena EP

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