RNA polymerase II carboxy-terminal domain kinases: emerging clues to their function. Eukaryot. Cell 1:153–162

The cloning of the largest subunit of RNA polymerase II (pol II) from mouse and Saccharomyces cerevisiae in 1985 (3, 28) revealed a remarkable and highly conserved domain known as the pol II carboxy-terminal domain (CTD). This domain has intrigued researchers interested in the mechanisms regulating gene expression ever since its discovery, because of both its simplicity and its complexity. The CTD is simple in the sense that it consists entirely of repeats of the 7-amino-acid consen-sus sequence YSPTSPS. The mouse (28) and human (125) CTDs consist of 52 repeats, of which 21 exactly match the consensus while 20 differ at only a single position (Table 1). This same consensus sequence is conserved in other eu-karyotes, with 27 repeats in budding yeast (18 exact and 5 with a single difference) (3) and 45 repeats in the more divergent Drosophila CTD (2 exact and 15 with a single difference) (4