Organization of the mammalian mitochondrial genome. Thirteen protein-coding genes (yellow), twenty-two tRNA genes (red) and two rRNA genes (orange) are encoded on a single circular nucleic acid and transcribed from three promoters (blue): LSP, HSP1 and HSP2, which are situated in a single region called the D-loop, which contains regulatory sequences that control transcription from all three promoters, including motifs for DNA-binding proteins such as Tfam. The inner circle of genes is encoded on the (-) strand and transcribed from the LSP promoter. The outer circle of genes is encoded on the (+) strand and transcribed from the HSP1 and HSP2 promoters. Transcription from HSP2 is terminated distal to the 16S rRNA gene. The resulting three polycistronic transcripts are processed by enzymatic excision of the tRNAs (red). ATP6, ATP8, subunits of ATP synthase F0; Cox1, Cox2, Cox3, subunits of cytochrome oxidase; CytB, cytochrome B, Nd1, Nd2, Nd3, Nd4, Nd4L, Nd5, Nd6, subunits of NADH dehydrogenase.