Analysis of the C-myc promoter-binding factor zf87/pur1 in transformed and nontransformed cell-lines.

ZF87/Pur1 is a zinc finger protein that binds to the purine-rich element ME1a1 within the c-myc P2 promoter. To better understand the effect of ZF87/Pur1 on c-myc gene expression, the gene was stably expressed in nontransformed NIH3T3 fibroblasts or was transiently overexpressed in transformed COS cells. The protein was targeted almost exclusively to the nucleus. In nuclear extracts from both cell types, the ectopically expressed ZF87/Pur1 protein generated a novel band in the gel-shift assay, due possibly to a modification since it was found that the protein was phosphorylated on serine and threonine in vivo. When the NIH3T3 cell lines constitutively expressing ZF87/Pur1 were analyzed under a variety of growth conditions, it was found that c-myc expression was not affected. In addition; overexpression of ZF87/Pur1 in COS cells did not affect transcription from the c-myc promoter in a cotransfection assay, but did increase the level of transcription from a promoter containing multiple ME1a1 binding sites. The data indicate that overexpression of ZF87/Pur1 alone is not enough to affect transcription from the c-myc P2 promoter; transcriptional activation from P2 may require additional limiting factors such as E2F.