The transcriptional apparatus of algal plastids

The plastid organelle of plants, eukaryotic algae and apicomplexan protists contains its own DNA genome (the 'plastome'). In higher plants, at least two RNA polymerases have been shown to participate in the transcription of plastid genes. The multisubunit 'plastid-encoded plastid RNA polymerase' (PEP) is derived from the cyanobacterial ancestor of the organelle and is structurally similar to eubacterial RNA polymerases. The core subunits of PEP are encoded by genes on the plastome, whilst sigma-like transcription factors and other regulatory factors are encoded in the nucleus. In contrast, the 'nuclear-encoded plastid RNA polymerase' (NEP) is a single-subunit enzyme related to the T3/T7 bacteriophage and mitochondrial RNA polymerases that is believed to have arisen by duplication of the nuclear gene for the mitochondrial enzyme. The situation in algal plastids is less well understood. All algal groups, apart from possibly the dinoflagellates, possess a PEP but it is far from clear whether any possess additional plastid RNA polymerases. In this article we review what is known about the transcription apparatus of algal plastids, and examine the evidence for and against the presence of NEPs.