4 Carboxy-terminal Domain of the Largest Subunit of Eukaryotic RNA Polymerase II

Abstract

OVERVIEW
The largest subunit of RNA polymerase II (RNA pol II) contains an unusual repetitive sequence. This carboxy-terminal domain (CTD) consists of tandem repeats of the consensus sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser. Although the function of this domain is unknown, the CTD is known to be phosphorylated at many sites. The timing of this modification suggests that the CTD may be involved in the transition from initiation to the elongation phase of transcription.

INTRODUCTION
The largest subunit of RNA pol II contains a unique repetitive carboxy-terminal extension that comprises tandem copies of the heptapeptide Tyr-Ser-Pro-Thr-Ser-Pro-Ser (Allison et al. 1985; Corden et al. 1985). This CTD is present in the largest subunit of RNA pol II from a variety of organisms but is not found in the corresponding large subunits of RNA pol I or III, or in prokaryotic RNA polymerase. Two forms of the largest subunit have been identified in cells. In one form, the CTD is multiply phosphorylated giving rise to the IIo subunit, which migrates more slowly than the unphosphorylated IIa form in SDS polyacrylamide gels (see Dahmus and Dynan, this volume). This shift between the unphosphorylated IIa and the phosphorylated IIo form of the largest subunit is thought to occur during the process of transcription initiation. As deduced by genetic studies, the integrity of the CTD is essential for cell viability; however, its mechanism of action is unknown. In this paper, we focus on our current understanding of the structure and function of this enigmatic polypeptide domain. The results of...