THE RNA POLYMERASE II C‐TERMINAL DOMAIN AND THE EVOLUTION OF RED ALGAE

Stiller, J. W. Department of Biology, East Carolina University, Greenville, NC 27858 USA In recent years a great deal of biochemical and genetic research relating to RNA polymerase II transcription has focused on the C‐terminal domain (CTD) of the pol II largest subunit (RPB1). In animals and yeast, where pol II function is most well characterized, this domain of tandemly repeated heptapeptides has been linked functionally to important steps in the initiation and processing of messenger RNA. Despite its essential role throughout the pol II transcription cycle, a tandemly repeated CTD has not been found in growing number of diverse eukaryotic organisms. Phylogenetic analyses of RPB1 sequences, excluding C‐terminal regions, show that those taxa in which the pol II CTD is strongly conserved make up a unique monophyletic group. The Rhodophyta is not a member of this “CTD‐clade,” nor are tandem heptad repeats present in most red algae. In addition, genetic complementation studies indicate that the evolutionary differences found in rhodophyte RPB1 C‐terminal sequences are incompatible with CTD‐based pol II transcription in yeast. The results of these combined analyses suggest that the CTD lies at the heart of a key innovation in the evolution of RNA polymerase II that helped to confer more intricate control over gene expression. The apparent absence of these control mechanisms in red algae may help to explain their lack of true tissue differentiation.