Binding of Tat to TAR RNA and CDK9/cyclin T1. Precipitation of biotin TAR RNA with purified Tat and with CDK9/cyclin T1. Lane 1, control denatured TAR RNA. Lane, control without Tat. Lane 3, untreated CDK9. cyclin T1. Lane 4, CDK9/cyclin T1 treated with PP2A (panel A) or with PP1 (panel B). Precipitated proteins and TAR RNA were recovered in SDS-loading buffer, resolved 12% SDS-PAGE and immunoblotted with indicated antibodies. Position of TAR RNA was determined by Ponceau-S staining.