ABSTRACT
All eukaryotic cells possess an extrachromosomal genetic system, sequestered in their mitochondria, which, though of limited informational content, plays a crucial role in the biogenesis of these organelles. 1,2 Mitochondrial DNA (mtDNA) has been known for some time to code for the RNA components of the organelle-specific protein synthesizing apparatus (rRNAs and tRNAs) and for a small set of essential proteins; these include some subunits of enzyme complexes of the inner mitochondrial membrane involved in respiration and oxidative phosphorylation — in particular the three largest subunits of cytochrome c oxidase (CO), cytochrome b and one or two subunits, depending upon the organism, of the oligomycin-sensitive ATPase — and, at least in yeast, a protein associated with the small ribosomal subunit. Furthermore, the recent discovery of unidentified reading frames (URFs) in the mtDNAs from several organisms has revealed a greater than previously suspected informational content of mtDNA, pointing to possible new functions of this DNA. 3,10
