Morphology of mitochondrial nucleoids in respiratory-deficient yeast cells varies depending on the unit length of the mitochondrial DNA sequence

Abstract

We investigated the morphology of mitochondrial nucleoids (mt-nucleoids) and mitochondria in Saccharomyces cerevisiae rho+ and rho- cells with DAPI staining and mitochondria-targeted GFP. Whereas the mt-nucleoids appeared as strings of beads in wild-type rho+ cells at log phase, the mt-nucleoids in hypersuppressive rho- cells (HS40 rho- cells) appeared as distinct punctate structures. In order to elucidate whether the punctate mt-nucleoids are common to other rho- cells, we observed the mt-nucleoids in rho- strains that retain different unit lengths of the mitochondrial DNA (mtDNA) sequence. As a result, rho- cells that have long mtDNA sequences, of more than 30 kb, had mt-nucleoids with a strings-of-beads appearance in tubular mitochondria. In contrast, rho- cells that have short mtDNA sequences, of >1 kb, had punctate mt-nucleoids in tubular mitochondria. This indicates that the morphology of mt-nucleoids in rho- cells significantly varies depending on the unit length of their mtDNA sequence. Analyses of mt-nucleoids suggest that the punctate mt-nucleoids in HS40 rho- cells consist of concatemeric mtDNAs and oligomeric circular mtDNAs associated with Abf2p and other nucleoid proteins.