Cyclin T1 domains involved in complex formation with Tat and TAR RNA are critical for tat-activation

Tat activates transcription from the human immunodeficiency virus type 1 (H IV-1) long terminal repeat (LTR) by increasing the processivity of RNA poly merase II. Recently, it has been demonstrated that the cellular kinase CDK9 and its binding partner cyclin T1 are involved in regulating transcription al elongation and tat-activation. Cyclin T1, CDK9 and Tat bind as a complex to elements in TAR RNA that are required for tat-activation. Here, we used cyclin T1 mutants to define domains in this protein that bind to both CDK9 and Tat and are involved in stimulating tat-activation. The region of cycl in T1 extending from amino acid residues 1 to 263 is necessary for complex formation with Tat bound to TAR RNA and for stimulation of tat-activation i n murine cells that are normally poorly responsive to the actions of Tat. I n contrast, a smaller region of cyclin T1 was required to bind to CDK9 and stimulate its kinase activity. Recombinant cyclin T1 and CDK9 stimulated bo th basal and tat-induced in vitro transcriptional elongation from the HIV-1 LTR. The effects of Tat on transcriptional elongation may be mediated by i ts ability to increase CDK9 phosphorylation of the RNA polymerase II C-term inal domain. These results demonstrate that cyclin T1 interactions with Tat and TAR RNA are critical for activation of HIV-1 gene expression. (C) 1999 Academic Press.