Okadaic acid modestly inhibits Tat-induced HIV-1 transcription in cultured cells. A, COS-7 cells were co-transfected without (lane 1) or with Tat-expressing vector and HIV-1 LTR-LacZ (lanes 2–10). Cells were also treated with indicated concentrations of okadaic acid (lanes 3–10). Expression of β-galactosidase was analyzed using ONPG-based assay. B, Quantification of the inhibition of Tat-induced transcription by okadaic acid using Prism. C, COS-7 cells were transfected with mutant HIV-1 LTR with a deletion of the fragment encoding TAR RNA (HIV-1 LTRΔTAR) without (lanes 1 and 3–10) or with Tat-expression plasmid (lane 2) and treated with the indicated concentrations of okadaic acid (lanes 3–10).