Mitochondrial transcription initiation: promoter structures and RNA polymerases

Abstract

A diversity of promoter structures It is evident that tremendous diversity exists between the modes of mitochondrial transcription initiation in the different eukaryotic kingdoms, at least in terms of promoter structures. Within vertebrates, a single promoter for each strand exists, which may be unidirectional or bidirectional. In fungi and plants, multiple promoters are found, and in each case, both the extent and the primary sequences of promoters are distinct. Promoter multiplicity in fungi, plants and trypanosomes reflects the larger genome size and scattering of genes relative to animals. However, the dual roles of certain promoters in transcription and replication, at least in yeast, raises the interesting question of how the relative amounts of RNA versus DNA synthesis are regulated, possibly via cis-elements downstream from the promoters. Mitochondrial RNA polymerases With respect to mitochondrial RNA polymerases, characterization of human, mouse, Xenopus and yeast enzymes suggest a marked degree of conservation in their behaviur and protein composition. In general, these systems consist of a relatively non-selective core enzyme, which itself is unable to recognize promoters, and at least one dissociable specificity factor, which confers selectivity to the core subunit. In most of these systems, components of the RNA polymerase have been shown to induce a conformational change in their respective promoters and have also been assigned the role of a primase in the replication of mtDNA. While studies of the yeast RNA polymerase have suggested it has both eubacterial (mtTFB) and bacteriophage (RPO41) orgins, it is not yet clear whether these characteristics will be conserved in the mitochondrial RNA polymerases of all eukaryotes. mtTFA-mtTFB; conserved but dissimilar functions With respect to transcription factors, mtTFA has been found in both vertebrates and yeast, and may be a ubiquitous protein in mitochondria. However, the divergence in non-HMG portions of the proteins, combined with differences in promoter structure, has apparently relegated mtTFA to alternative, or at least non-identical, physiological roles in vertebrates and fungi. The relative ease with which mtTFA can be purified (Fisher et al. 1991) suggests that, where present, it should be facile to detect. mtTFB may represent a eubacterial sigma factor adapted for interaction with the mitochondrial RNA polymerase. In plants, sigma-like factors capable of interacting with a eubacterial polymerase are found in chloroplasts (Lerbs et al. 1988; Tiller and Link 1993; Troxler et al. 1994) raising the possibility that a gene family in plants contributes transcriptional factors to both mitochondria and chloroplasts. In coming years, we can expect a more detailed analysis of RNA polymerases, accessory factors and promoter structures, which will lead to a better understanding of the different modes of mitochondrial transcription initiation in eukaryotic species and the evolutionary relationships between them.