The transcriptional inhibitors, actinomycin D and alpha-amanitin, activatethe HIV-1 promoter and favor phosphorylation of the RNA polymerase IIC-terminal domain

Actinomycin D and alpha-amanitin are commonly used to inhibit transcription . Unexpectedly, however, the transcription of the human immunodeficiency vi rus (HIV-1) long terminal repeats (LTR) is shown to be activated at the lev el of elongation, in human and murine cells exposed to these drugs, whereas the Rous sarcoma virus LTR, the human cytomegalovirus immediate early gene (CMV), and the HSP70 promoters are repressed. Activation of the HIV LTR is independent of the NF kappa B and TAR sequences and coincides with an enha nced average phosphorylation of the C-terminal domain (CTD) from the larges t subunit of RNA polymerase II. Both the HIV-1 LTR activation and the bulk CTD phosphorylation enhancement are prevented by several CTD kinase inhibit ors, including 5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole. The effic acies of the various compounds to block CTD phosphorylation and transcripti on in vivo correlate with their capacities to inhibit the CDK9/ PITALRE kin ase in vitro. Hence, the positive transcription elongation factor, P-TEFb, is likely to contribute to the average CTD phosphorylation in vivo and to t he activation of the HIV-1 LTR induced by actinomycin D.