DIFFERENTIAL PROTEIN-BINDING TO THE C-MYC PROMOTER DURING DIFFERENTIATION OF HEMATOPOIETIC-CELL LINES

In vivo footprinting has been used to examine protein binding sites in the c-myc promoter during differentiation cell lines. The c-myc gene is proliferating cells, but c-myc levels decrease dramatically during differentiation. A number of potential protein binding sites have been identified from in vitro studies of the c-myc promoter, but very litt le is known about occupancy of these sites in vivo. We have identified in vivo footprints at DNase hypersensitive sites II2 and III1 which d isappear during differentiation, while a footprint at site IV is prese nt only in differentiated cells. Footprints at DNase hypersensitive si tes I and II1 do not change with differentiation. A protected region n ear DNase hypersensitive site III2 is present in both undifferentiated and differentiated cells, but it extends further 5' in undifferentiat ed cells. From the protected sequences we have been able to identify c andidate transcription factors likely to be involved in the control of c-myc expression. By electrophoretic mobility shift assay we have dem onstrated that a protein binds to the sequence at site IV. We have als o examined the 3' end of the first exon and the 5' end of intron I and do not find any evidence for protein binding sites in this region tha t was thought to be important for the block to transcription elongatio n during differentiation.