Tat and trans-activation-responsive (TAR) RNA-independent induction of HIV-1 long terminal repeat by human and murine cyclin T1 requires Sp1.

P-TEFb, cyclin T1 + CDK9, is needed for the expression of cellular promoters and primate lentiviral long terminal repeats (LTRs). Curiously, cellular and lentiviral promoters differ dramatically in the requirements for positive transcriptional elongation factor (P-TEF) b activity. Lentiviral LTRs, but not cellular promoters, need an RNA-associated P-TEFb/Tat/TAR (trans-activation-responsive) RNA ternary complex. Ternary complex defective murine cycT1 is apparently inactive for lentiviral transcription. Why P-TEFb requires Tat/TAR for LTRs but not for cellular promoters remains unknown. To explore this question, we sought to determine whether DNA targeting of murine and human cyclin T1 can reconstitute a Tat/TAR-independent activity to the HIV-1 LTR. In the absence of Tat and TAR, we found that both HuCycT1 and MuCycT1 can robustly activate the HIV-1 LTR. We further showed that Sp1 is necessary and sufficient for this DNA-targeted activity. Thus, like cellular promoters, HIV-1 LTR can use P-TEFb function without a Tat/TAR RNA complex. This activity could explain recent findings of robust HIV-1 replication in rat cells that cannot form a P-TEFb/Tat/TAR moiety.