Cell cycle-dependent stimulation of the HIV-1 promoter by Tat-associated CAK activator

Activation of the HIV-I promoter by the virally encoded Tat protein is char acterized by efficient processive transcription, mediated by host cell fact ors that are tethered to the promoter with the Tat-TAR RNA complex. Importa ntly, viral gene activation has been shown to be stimulated in mitogenicall y induced cells, although the link between cell cycle regulation and viral gene activation is unclear. We reported a Tat-associated CAK/CTD kinase fro m mitogenically induced primary human T-cells (TTK) (S. Nekhai et al., 1997 , J. Virol. 71, 7436-7441). Here, biological activity of the kinase has bee n studied by direct microinjection at the individual-cell level. The TTK-de pendent Tat response is maximal during G1 phase as shown by co-injection wi th Tat protein in cells synchronized at the various stages of the cell cycl e. The cell cycle dependence of the Tat response was confirmed by inhibitin g G0 --> G1 progression with the expression of dominant negative mutant Ras (Asn17) or the cyclin-dependent kinase CDK4. The results support a mechanis m whereby transactivation of the HIV promoter is regulated by cell growth s ignal transduction pathways that target the Tat cofactor, (C) 2000 Academic Press.