FIGURE 9-21 Model of antitermination complex composed of HIV Tat protein and several cellular proteins. The TAR element in the HIV transcript contains sequences recognized by Tat and the cellular protein cyclin T. Cyclin T activates and helps position the protein kinase CDK9 near its substrates, the CTD of RNA polymerase II, NELF, and DSIF. CTD phosphorylation at serine 2 of the Pol II CTD heptad repeat is required for transcription elongation. Cellular proteins DSIF and the NELF complex are also involved in regulating Pol II elongation, as discussed in the text. See T. Wada et al., 1998, Gene Dev. 12:343; Y. Yamaguchi et al., 1999, Cell 97:451; and T. Yamada et al., 2006, Mol. Cell 21:227.