EFFECT OF THE POSITION OF TAR ON TRANSCRIPTIONAL ACTIVATION BY HIV-1 TAT IN-VIVO

Efficient expression of the human immunodeficiency virus (HIV) genome requires the viral-encoded transactivator Tat. Tat interacts with the highly structured trans-activation-response (TAR) RNA that is found at the 5' end of all viral transcripts, and mediates the formation of tr anscription complexes that are capable of elongation through the entir e length of the viral genome. By placing TAR immediately downstream fr om the P2 promoter of the mouse c-myc gene, we have previously shown t hat Tat can also direct transcriptional elongation through potential s ites of premature termination within c-myc in transfected HeLa cells. We now demonstrate that Tat can activate c-myc transcription when TAR is positioned internally within the c-myc transcript at distances up t o 353 nt downstream from the P2 promoter. We show that Tat can also ac tivate transcription from the c-myc P1 promoter, which is located 165 nt upstream from P2 in these hybrid gene constructs. These novel findi ngs show that Tat can activate transcription in vivo when TAR is posit ioned at distances up to 518 nt downstream from the site of transcript ional initiation. The ability of TAR to mediate Tat-activated transcri ption over distances greater than previously appreciated has important implications for the mechanism of action of Tat. (C) 1996 Academic Pr ess Limited