bMitochondrial DNA (mtDNA) is highly compacted into DNA-protein structures termed mitochondrial nucleoids (mt-nucleoids). The key mt-nucleoid components responsible for mtDNA condensation are HMG box-containing proteins such as mammalian mitochondrial transcription factor A (TFAM) and Abf2p of the yeast Saccharomyces cerevisiae. To gain insight into the function and organization of mt-nucleoids in strictly aerobic organisms, we initiated studies of these DNA-protein structures in Yarrowia lipolytica. We identified a principal component of mt-nucleoids in this yeast and termed it YlMhb1p (Y. lipolytica mitochondrial HMG box-containing protein 1). YlMhb1p contains two putative HMG boxes contributing both to DNA binding and to its ability to compact mtDNA in vitro. Phenotypic analysis of a ⌬mhb1 strain lacking YlMhb1p resulted in three interesting findings. First, although the mutant exhibits clear differences in mt-nucleoids accompanied by a large decrease in the mtDNA copy number and the number of mtDNA-derived transcripts, its respiratory characteristics and growth under most of the conditions tested are indistinguishable from those of the wild-type strain. Second, our results indicate that a potential imbalance between subunits of the respiratory chain encoded separately by nuclear DNA and mtDNA is prevented at a (post)translational level. Third, we found that mtDNA in the ⌬mhb1 strain is more prone to mutations, indicating that mtHMG box-containing proteins protect the mitochondrial genome against mutagenic events. Individual eukaryotic cells contain a population of mitochondrial DNA (mtDNA) molecules, the number of which may reach several thousand copies. For example, aerobically grown diploid cells of the yeast Saccharomyces cerevisiae contain, on average, 100 molecules of 85-kbp mtDNA. With a distance of 0.34 nm between base pairs in DNA, the total length of mtDNA reaches almost 3 mm per cell, while the diameter of the cell does not exceed 3 to 4 m. Analogously to its nuclear counterpart, mtDNA must be packaged into condensed nucleoprotein structures termed mitochondrial nucleoids (mt-nucleoids) (1-6). The size of mt-nucleoids in S. cerevisiae ranges from 0.2 to 0.9 m, meaning that mtDNA in yeasts undergoes compaction of roughly 3 orders of magnitude. Although it is known that the size and shape (oval versus globular) of mt-nucleoids, the number of mt-nucleoids (ranging from 50 to 70) per diploid cell (2), and the number of copies (up to 9) of mtDNA per mt-nucleoid (3) in S. cerevisiae depend on physiological conditions, the molecular mechanisms mediating nucleoid maintenance, dynamics, and roles in mtDNA distribution/segregation during cell division are largely not understood. This is also true for mammalian mt-nucleoids, which, in contrast to their yeast counterparts, contain a relatively small number of mtDNA molecules and are thus more solitary in nature (7,8). Description of these intra-and interspecific differences in the organization of mt-nucleoids would greatly facilitate our understanding of the m...

Mdm31p is an inner mitochondrial membrane (IMM) protein with unknown function in Saccharomyces cerevisiae. Mutants lacking Mdm31p contain only a few giant spherical mitochondria with disorganized internal structure, altered phospholipid composition and disturbed ion homeostasis, accompanied by increased resistance to the electroneutral K + /H + ionophore nigericin. These phenotypes are interpreted as resulting from diverse roles of Mdm31p, presumably in linking mitochondrial DNA (mtDNA) to the machinery involved in segregation of mitochondria, in mediating cation transport across IMM and in phospholipid shuttling between mitochondrial membranes. To investigate which of the roles of Mdm31p are conserved in ascomycetous yeasts, we analysed the Mdm31p orthologue in Schizosaccharomyces pombe. Our results demonstrate that, similarly to its S. cerevisiae counterpart, SpMdm31 is a mitochondrial protein and its absence results in increased resistance to nigericin. However, in contrast to S. cerevisiae, Sz. pombe cells lacking SpMdm31 are also less sensitive to the electrogenic K + ionophore valinomycin. Moreover, mitochondria of the fission yeast mdm31Δ mutant display no changes in morphology or phospholipid composition. Therefore, in terms of function, the two orthologous proteins appear to have considerably diverged between these two evolutionarily distant yeast species, possibly sharing only their participation in ion homeostasis.

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