Schizosaccharomyces pombe: A Short Review of a Short Mitochondrial Genome

Abstract

The sizes and genomic organizations of mtDNAs from sea urchins, insects, toads, and humans are virtually the same. An entirely different situation is found among ascomycetes, where mitochondrial genomes differ greatly in size and organization. This divergence is also found in yeasts, where the extremes are marked by the long mitochondrial genome of Saccharomyces cerevisiae and the short ones of Torulopsis glabrata and Schizosaccharomyces pombe. Both S. cerevisiae and T. glabrata can survive without mtDNA, whereas S. pombe seems to be dependent on mtDNA even under fermentative conditions. We have focused our interest on the characterization of the small mitochondrial genome of S. pombe, since the study of extremely short genomes may provide insight into the minimal requirement of a functional genetic unit and the evolutionary mtDNA.

EXTRANUCLEAR DNA MOLECULES
Electron microscopy of DNA molecules from mitochondrial fractions revealed the presence of three molecular species of approximately 2-, 3-, and 6-μm contour length (Fig. 1). After treatment of the mitochondrial fraction with DNase, only the 6-μm molecular species remained. This indicates that these molecules are indeed mtDNA. The roles of the two other DNA species are still uncertain. The 3-μm species could be homologous to circular molecules in S. cerevisiae and T. glabrata, which have been shown to contain sequences hybridizing to cytoplasmic rRNAs.

REPLICATION OF mtDNA
As a consequence of inhibition of cytoplasmic protein synthesis, nuclear DNA synthesis ceases immediately in S. cerevisiae; mtDNA synthesis, however, proceeds for several hours. In contrast, blocking protein synthesis on cytoplasmic ribosomes in...