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

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Abstract
The HIV‐1‐encoded Tat protein controls transcription elongation by increasing processivity of RNA polymerase II (Pol II). Here, we have identified a Tat 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 unidentified polypeptides, but none of the SRB/MED proteins or other factors found associated with the previously described RNA Pol II holoenzyme complex. Tat‐SF supports basal, Sp1‐activated and Tat‐activated transcription in a reconstituted system, and a Tat‐SF‐derived fraction lacking RNA Pol II can complement non‐responsive RNA Pol II complexes for Tat‐enhanced HIV‐1 transcription, indicating that Tat‐SF contains factors that are critical for Tat function. Both Tat‐SF and RNA Pol II holoenzyme are present in HeLa nuclear extracts and each can be recruited to the HIV‐1 promoter. Our results indicate that Tat‐SF is a Tat cofactor‐containing RNA Pol II complex whose recruitment to the promoter provides elongation factors important for Tat‐enhanced HIV‐1 transcription following TAR RNA synthesis.