Glial cell-specific regulation of the JC virus early promoter by large T antigen

Progressive multifocal leukoencephalopathy (PML) is a fatal demyelinating d isease that results from an oligodendrocyte infection caused by JC virus. T he JC virus early promoter directs cell-specific expression of the viral re plication factor large T antigen, and thus transcriptional regulation const itutes a major mechanism of glial tropism in PML. We have previously demons trated that T antigen controls the JC virus basal promoter in a glial cell- specific manner, since T antigen repressed the JC virus and simian virus 40 (SV40) early promoters in glioma cells but induced strong activation of th e JC virus early promoter in nonglial cells. To further analyze these findi ngs, T antigen and nuclear extracts from glial and nonglial cells were used to examine DNase I footprints on the proximal promoter. T-antigen binding to site II was more extensive than expected based on sequence homology with SV40, and nuclear proteins protected several regions of the proximal promo ter in a cell-specific manner. Multiple Sp1 binding domains were identified . Site-directed mutagenesis revealed that T-antigen-mediated activation req uired a TATA box sequence, a pentanucleotide repeat immediately upstream of the TATA box, and an Sp1 binding site downstream of the TATA box, When foo tprints were obtained with mutant promoters which blocked T-antigen-induced transactivation, no change in T-antigen binding was observed. These result s suggest that T antigen activates the JC virus basal promoter in nonglial cells by interaction with the transcription initiation complex.