FIGURE 12-10 Proteins encoded in mitochondrial DNA and their involvement in mitochondrial processes. Only the mitochondrial matrix and inner membrane are depicted. Most mitochondrial components are encoded by the nucleus (blue); those highlighted in pink are encoded by mtDNA in some eukaryotes but by the nuclear genome in other eukaryotes, whereas a small portion are invariably specified by mtDNA (orange). Mitochondrial processes that have exclusively nucleus-encoded components are listed at the top. Complexes I–V are involved in electron transport and oxidative phosphorylation. Tim, Sec, Tat, and Oxa1 translocases are involved in protein import and export and in the insertion of proteins into the inner membrane (see Chapter 13). RNase P is a ribozyme that processes the 5′ end of tRNAs (discussed in Chapter 10). It should be noted that the majority of eukaryotes have a multisubunit complex I as depicted, with three subunits invariantly encoded by mtDNA. However, in a few organisms (Saccharomyces, Schizosaccharomyces, and Plasmodium), this complex is replaced by a nucleus-encoded, single-polypeptide enzyme. See G. Burger et al., 2003, Trends Genet. 19:709.