A novel RNA polymerase II-containing complex potentiates Tat-enhanced HIV-1 transcription

The HIV-1-encoded Tat protein controls transcription elongation by increasi ng processivity of RNA polymerase II (Pol II). Here, we have identified a T at stimulatory activity (Tat-SF) as a novel RNA Pol II-containing complex. Remarkably, Tat-SF contains the previously identified Tat cofactors Tat-SF1 , P-TEFb and hSPT5/Tat-CT1, in addition to RNA Pol II and other unidentifie d polypeptides, but none of the SRB/MED proteins or other factors found ass ociated with the previously described RNA Pol II holoenzyme complex. Tat-SF supports basal, Spl-activated and Tat-activated transcription in a reconst ituted system, and a Tat-SF-derived fraction lacking RNA Pol II can complem ent non-responsive RNA Pol II complexes for Tat-enhanced HIV-1 transcriptio n, indicating that Tat-SF contains factors that are critical for Tat functi on. Both Tat-SF and RNA Pol II holoenzyme are present in HeLa nuclear extra cts and each can be recruited to the HIV-1 promoter. Our results indicate t hat Tat-SF is a Tat cofactor-containing RNA Pol II complex whose recruitmen t to the promoter pro,ides elongation factors important for Tat-enhanced HI V-1 transcription following TAR RNA synthesis.