Domains in the SPT5 protein that modulate its transcriptional regulatory properties

SPT5 and its binding partner SPT4 regulate transcriptional elongation by RN A polymerase II, SPT4 and SPT5 are involved in bath 5,6-dichloro-1-beta-D-r ibofuranosylbenzimidazole (DRB)-mediated transcriptional inhibition and the activation of transcriptional elongation by the human immunodeficiency vir us type 1 (HIV-1) Tat protein. Recent data suggest that P-TEFb, which is co mposed of CDK9 and cyclin T1, is also critical in regulating transcriptiona l elongation by SPT4 and SPT5, In this study, we analyze the domains of SPT 5 that regulate transcriptional elongation in the presence of either DRB or the HIV-1 Tat protein. We demonstrate that SPT5 domains that bind SPT4 and RNA polymerase II, in addition to a region in the C terminus of SPT5 that contains multiple heptad repeats and is designated CTR1, are critical for i n vitro transcriptional repression by DRB and activation by the Tat protein . Furthermore, the SPT5 CTR1 domain is a substrate for P-TEFb phosphorylati on. These results suggest that C-terminal repeats in SPT5, like those in th e RNA polymerase II C-terminal domain, are sites for P-TEFb phosphorylation and function in modulating its transcriptional elongation properties.