Transcription and Transcription Regulation in Chloroplasts and Mitochondria of Higher Plants

The mitochondrial and plastid transcription machineries of plants are highly complex and reflect the eukaryotization of the genetic systems of the organelles. Plastid transcription relies on two types of different RNA polymerases (RNAPs): one plastid-encoded eubacterial-type polymerase (PEP) and at least one nuclear-encoded phage-type RNAP (NEP). PEP uses σ70-like promoters, whereas NEP recognizes promoter sequences resembling mitochondrial promoter motifs. Mitochondrial transcription, too, is performed by at least one nuclear-encoded phage-type RNA polymerase. Mitochondrial promoters transcribed by this RNAP are variable and only a subset can be distinguished by conserved sequence motifs. The organellar phage-type RNAPs are encoded by a small nuclear gene family that consists of three RpoT genes in eudicot and two RpoT genes in monocot plants. While the plastid PEP is complemented by nuclear-encoded σ-factors to confer promoter-specific binding, the principal specificity factors that interact with the mitochondrial and plastid (NEP) RpoT polymerases have not been identified yet.