Effect of Tat on transcription of the provirus. In agreement with the results in Figure 4, expression of Tat alone exerted minimal stimulatory effect on the 5'LTR (Figures 7A-7C, Tat). However, in combination with TNF-, Tat expression activated viral transcription greatly (70-80 of GFP-expressing cells) in all three cell lines (Figures 7A-7C, Tat + TNF-). We further investigated the ability of PM