A ROLE FOR SP AND HELIX-LOOP-HELIX TRANSCRIPTION FACTORS IN THE REGULATION OF THE HUMAN ID4 GENE PROMOTER ACTIVITY

Id family helix-loop helix (HLH) proteins are involved in the regulati on of proliferation and differentiation of several cell types. To iden tify cis and trans-acting factors that regulate Id4 gene expression, w e have analyzed the promoter regulatory sequences of the human Id4 gen e in transient transfections and gel mobility shift assays. We have id entified two functional elements, both located downstream from the TAT A motif, that control Id4 promoter activity. One element contains a co nsensus E-box, and we demonstrated that the protein complex binding to the E-box contains the bHLH-zip upstream stimulatory factor (USF) tra nscription factor. Enforced expression of USF1 leads to E-box-mediated stimulation of promoter activity. The E-box also mediated stimulatory effects of several bHLH transcription factors, and co expression of I d4 blocked the stimulatory effect mediated by the bHLH factors. A seco nd element is a GA motif, located downstream from the transcriptional start sites, mutation of which resulted in a 20-fold increase in trans criptional activity. Gel-shift analysis and transfections into Drosoph ila Schneider SL2 cells showed that the repressor element is recognize d by both Sp1 and Sp3 factors. These data suggest that Id4 transcripti on control is highly complex, involving both negative and positive reg ulatory elements, including a novel inhibitory function exerted by Sp1 and Sp3 transcription factors.