Transcriptional activation by the parvoviral nonstructural protein NS-1 is mediated via a direct interaction with Sp1

The nonstructural protein NS-1, encoded by the parvovirus minute virus of mice, is a potent regulator of viral gene expression. NS-1 does not bind DNA in a sequence-specific manner, and the mechanism by which it modulates viral promoter function is unclear. We have used Gal4-NS-1 fusion protein constructs to identify and characterize an activating domain encoded within the C-terminal 88 amino acids of NS-1 which competes effectively with the acidic activator domain of the herpes simplex virus VP16 protein. DNA affinity chromatography and immunoprecipitation experiments demonstrate that protein-protein interactions between the transcription factor Sp1 and NS-1 are required to bind NS-1 to promoter DNA in vitro. Cotransfection of Gal4-NS-1 and Sp1-VP16 acidic activator constructs into Drosophila melanogaster Schneider cells, which lack endogenous Sp1, stimulates transcription from a minimal promoter containing five Gal4 binding sites, while single-construct transfections do not. Cotransfection of Schneider cells with wild-type NS-1 and Sp1 constructs activates transcription from a simian virus 40 promoter 10- to 30-fold over that of either construct alone. Thus, Sp1-NS-1 interactions in vivo can stimulate transcription from a heterologous promoter containing Sp1 binding sites.