FIGURE 1.

Primary structure of the CTD of pol II. (A) The primary sequence of the 52-repeat human CTD is shown, with the heptapeptide repeat numbers indicated. Residues that do not follow the consensus (YSPTSPS) are highlighted in black. A schematic representation of the CTDs of the different pol II expression plasmids used is also shown, where consensus and degenerate CTD repeats are indicated as light and dark-shaded boxes, respectively. The white box located at the very C-terminus of the CTD represents its terminal 10 amino acid (nonheptapeptide repeat) sequence (see Introduction). The expression plasmids each contain an N-terminal Flag-epitope and an α-amanitin-resistant pol II catalytic domain fused to a CTD with the following heptapeptide repeat composition: 1-52 (pol II 1-52; wild type), repeats 1-22 (pol II 1-22), repeats 31-52 (pol II 31-52), two tandem blocks of repeats 1-22 (pol II CC), or fusion of repeats 31-52 to 1-22 (pol II DC). (B) Expression of mutant pol II derivatives containing different CTDs. Each pol II expression plasmid shown in (A) was transfected into human 293 cells, and the corresponding protein lysates were immunoblotted with the α-Flag antibody.