Coordinate effects of human immunodeficiency virus type 1 protein Tat and cellular protein Pur alpha on DNA replication initiated at the JC virus origin

JC virus (JCV) causes progressive multifocal leukoencephalopathy, a demyeli nating disease in brains of individuals with AIDS. Previous work has shown that the Tat protein, encoded by human immunodeficiency virus type 1 (HIV-1 ), can interact with cellular protein Pur alpha to enhance bath TAR-depende nt HIV-1 transcription and JCV late gene transcription. Tat has been shown to activate JCV transcription through interaction with Pur alpha, which bin ds to promoter sequence elements near the JCV origin of replication. DNA fo otprinting has shown that Pur alpha and large T-antigen cooperatively inter act at several binding sites in the origin and transcriptional control regi on. Overexpression of Pur alpha inhibits replication initiated at the JCV o rigin by T-antigen. In transfected glial cells Tat reversed this inhibition and enhanced DNA replication. In an in vitro replication system maximal ac tivation by Tat, more than sixfold the levels achieved with T-antigen alone , was achieved in the presence of Pur alpha, Effects of mutant Tat proteins on both activation of replication and binding to Pur alpha have revealed t hat Cys22 exerts a conformational effect that affects both activities. The origin of an archetypal strain of JCV was less susceptible to activation of replication by Tat relative to the rearranged Mad-1 strain. These results have revealed a previously undocumented role for Tat in DNA replication and have indicated a regulatory role for JCV origin auxiliary sequences in rep lication and activation by Tat.