Mitochondrial transcription: is a pattern emerging?

Despite the striking similarities of RNA polymerases and transcription signals shared by eubacteria, archaebacteria and eukaryotes, there has been little indication that transcription in mitochondria is related to any previously characterized model. Only in yeast has the subunit structure of the mitochondrial RNA polymerase been determined. The yeast enzyme is composed of a core related to polymerases from bacteriophage T7 and T3, and a promoter recognition factor similar to bacterial sigma factors. Soluble systems for studying mitochondrial transcript initiation in vitro have been described from several organisms, and used to determine consensus sequences at or near transcription start sites. Comparison of these sequences from fungi, plants, and amphibians with the T7/T3 promoter suggests some intriguing similarities. Mammalian mitochondrial promoters do not fit this pattern but instead appear to utilize upstream sites, the target of a transcriptional stimulatory factor, to position the RNA polymerase. The recent identification of a possible homologue of the mammalian upstream factor in yeast mitochondria may indicate that a pattern will eventually be revealed relating the transcriptional machineries of all eukaryotic mitochondria.