Figure 3.

Tat specifically inhibits transcription from the MHC class I promoter. (A) In vitro transcription of the MHC class I promoter construct 313CAT was performed in the presence or absence of 0.2 μg of recombinant proteins, GST-Tat₁₀₁, GST-Tat_1–67, GST-Tat_67–101, or GST-SNAP23. Transcripts were detected by primer extensions, as detailed in Materials and Methods. Shown is a representative autoradiogram. (B) Titration of the effects of Tat₁₀₁, Tat_67–101 fragment, and GST on in vitro transcription of the class I promoter. Tat ₁₀₁ and Tat_67–101 were both added as GST fusion proteins. The results are plotted relative to the level of transcription in the absence of added protein. The data shown in A and B are from separate experiments. (C) mTAF_II250 fragment relieves Tat-mediated repression of transcription. In vitro transcription reactions of 313CAT were performed in the presence and absence of 0.5 μg Tat₁₀₁ and increasing concentrations of either mTAFII250 fragment (open symbol) or control SNAP23 protein (closed symbol). The magnitude of Tat repression was determined at each concentration of competitor.