THE EVOLUTION OF RNA POLYMERASE II IN RED ALGAE

Whether red algae are the sister group to green plants has been an intriguing question for over a century. We are investigating both the evolutionary and functional significance of differences in DNA‐dependent RNA polymerase II between rhodophytes and other eukaryotic organisms. Results of both phylogenetic and genetic complementation studies are consistent with an origin of the Rhodophyta that was independent of the common ancestor of green plants, animals, fungi and several related protistan groups. Examination of long‐branch attraction among Pol II gene sequences indicates that this separate emergence of red algae is not the result of an artifactual grouping of rhodophytes with more rapidly evolving taxa. The absence in red algal pol II largest subunits of a canonical C‐terminal domain (CTD), an essential component of mRNA transcription in plants, animals and fungi, further supports the proposition that red algae diverged independently of these groups, and before CTD function was incorporated fully into Pol II function. Evolutionary complementation, that is, in vivo replacement of the wild‐type CTD from yeast with RPB1 C‐terminal sequences from a red alga, further supports the hypothesis that fundamental differences exist in Pol II function between red algae and organisms with a CTD‐based mRNA transcription cycle.