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SEQUENCES OF THE LARGEST SUBUNIT OF RNA POLYMERASE II FROM TWO RED ALGAE AND THEIR IMPLICATIONS FOR RHODOPHYTE EVOLUTION

The Rhodophyta are a widespread group of multicellular eukaryotes that occupy a range of habitats and exhibit a broad variety of morphologies and life histories. Unlike green plants, animals, and even brown algae, the red algae have attained this diversity without having evolved true tissue differentiation. Moreover, the relationship of red algae to other multicellular lineages remains controversial, and the molecular and biochemical mechanisms of their development are largely unexplored. We have isolated the 3′ end of the gene encoding the largest subunit (RPB1) of RNA polymerase II (Pol II) from two divergent red algae, Porphyra yezoensis Ueda and Bonnemaisonia hamifera Heriot. The Pol II largest subunit lies at the catalytic center of the enzyme responsible for mRNA transcription in eukaryotic cells. In plants, animals, and fungi, the 3′ distal region of the RPB1 gene encodes a carboxyl‐terminal domain (CTD) that is the organizing center for assembly of the RNA Pol II holoenzyme essential for developmentally regulated transcription. Examination of rhodophyte RPB1 C‐terminal regions as well as phylogenetic analyses based on conserved gene sequences suggest that the red algae emerged in the course of eukaryotic evolution before the CTD had been codified in the remaining crown taxa. The possibility that a causal relationship underlies the correlated lack of a CTD and absence of tissue differentiation in red algae is proposed as an intriguing subject for further study.