Molecular Genetics

Facts

• Heteroplasmic yeast cells tend to homoplasmy after a few generations. The number of generations is many fewer than expected considering each DNA genome as inherited independently.
• R- mutants of yeast have deletions of various parts of the mitochondrial DNA and may have remaining sections repeated. The r- mutants cause a small colony "petite" phenotype because of the absence of respiration.
• Some r- mutants (hypersuppressive) are dominant to wild type r+ mitochondrial DNA.
• A mutation in a gene, MGT, causes reversal of hypersuppression (ref). The gene encodes a mitochondrially located, cruciform-cutting endonuclease that resolves Holiday intermediates.
• In mgt mutants, nucleoids ("chondriolites") are smaller and more numerous than in wild type cells.
• In heteroplasmic yeast cells, recombination between mitochondrial genomes occurs at high frequency as detected, for example, by the appearance of novel restriction fragments.

Interpretations

• The segregating unit of mitochondrial DNA is not a single mitochondrial DNA, but a complex of DNAs interconnected as recombination intermediates.
• The products of mitochondrial DNA replication must remain closely associated so that they may form recombination intermediates and segregate as one unit.
• Recombination is frequent in yeast mitochondria (ref).
• Heteroplasmic states are disfavored.

Further information

• At least 22 genes play roles in the segregation of mitochondria and mitochondrial genomes in to daughter cells of brewer's yeast.
• Random segregation can account for variegation.
• r- mutants often contain multiple origins of replication (due to the duplication of segments) and may thus replicate more often (ref). Such replication may contribute to rapid progression to homoplasmy.
• Recombination can be accounted for by organelle fusion.

This is page 1376 of Molecular Genetics by Ulrich Melcher, © 1997-8, 2001, 2003-4