Fig. 2. Transcriptional regulators of a typical nuclear-encoded respiratory gene. Mitochondrial genes typically lack TATA boxes and constitutive expression is usually mediated by the specificity protein 1 (Sp1) family of transcription factors. The promoters for mitochondrial genes often possess binding sites for CREB, PPAR, NRF-1 and NRF-2. The members of the mammalian PPAR and RxR families are shown but the extent of the family in other vertebrates is not yet established. Not every tissue expresses each of the RxR and PPAR family members. Hexagons represent the specific agonists for the nuclear hormone receptors.