Immunoblot analysis of untreated HeLa NE and NC2β-depleted HeLa NE (NE[ΔNC2β]). Equal amounts of NE protein (30 μg) and 10 μl of anti-NC2β column eluate were subjected to 4–16% gradient SDS/PAGE, transferred to nitrocellulose, and analyzed with antibodies to the proteins indicated. Various antibodies were used to test for the presence of the large RPB1 subunit of RNAP II. mAb 8WG16 (Babco, Richmond, CA) is specific for the CTD and detects mainly the unphosphorylated, CTD-containing RNAP IIA form (lanes 7–9). The polyclonal antibody N-20 (Santa Cruz Biotechnology) recognizes an epitope located at the N terminus of RPB1 and therefore detects CTD-containing (IIA, IIO) and CTD-less (IIB) RPB1 (lanes 10–12). mAbs H5 and H14 (Babco) detect only the phosphorylated IIO form of the CTD and are specific for phosphoserine 2 (H5; lanes 13–15) and phosphoserin 5 (H14; lanes 16–18).