THE TAX PROTEIN OF HUMAN T-CELL LEUKEMIA-VIRUS TYPE-1 MEDIATES THE TRANSACTIVATION OF THE C-SIS PLATELET-DERIVED GROWTH FACTOR-B PROMOTER THROUGH INTERACTIONS WITH THE ZINC-FINGER TRANSCRIPTION FACTORS SP1 ANDNGFI-A/EGR-1/

Transcriptional up-regulation of the c-sis/platelet-derived growth fac tor-B (PDGF-B) proto-oncogene by the Tax protein of human T cell leuke mia virus type 1 has been implicated as one possible mechanism of cell ular transformation by human T-cell leukemia virus type 1. In previous work, we identified an essential site in the c-sis/PDGF-B promoter, T ax-responsive element 1 (TRE1), necessary for transactivation by Tax. We also identified Sp1, Sp3, and NGFI-AIEgr-1 as the primary nuclear t ranscription factors binding to TRE1 which mediate Tax responsiveness. In the present work, we have investigated the mechanism(s) whereby Ta x transactivates the c-sis/PDGF-B proto-oncogene. In vitro transcripti on assays showed that Tax was able to significantly increase the trans criptional activity of a template containing the -257 to +74 region of the c-sis/ PDGF-B promoter. Electrophoretic mobility shift assay anal ysis showed that Tax increased the DNA binding activity of both Spl an d NGFI-A/Egr-1 using a TRE1 probe. Analysis of Tax mutants showed that two mutants, IEXC29S and IEXL320G, were unable to significantly trans activate the c-sis/PDGF-B promoter. Finally, co-immunoprecipitation an alysis revealed that Tax is able to stably bind to both Spl and NGFI-A /Egr-1. Interestingly, co-immunoprecipitation analysis also revealed t hat Tax mutant IEXC29S is unable to interact with NGFI-A/Egr1, whereas Tax mutant IEXL320G is able to interact with NGFI-A/Egr-1.