IDENTIFICATION OF A 2ND PROMOTER IN THE HUMAN C-MYB PROTOONCOGENE

We have previously described an alternatively spliced cDNA clone of th e human c-myb proto-oncogene which has been shown to enhance the diffe rentiation of Friend murine erythroleukemia cells. This clone, pMbm-2, contains unique 5' sequences which replace exon 1. The human c-myb in tron 1 was sequenced to determine the exact position of this unique se quence and to further characterize the role of intron 1 in the regulat ion of the human c-myb gene. Here we report that intron 1 of c-myb is highly conserved between human and mouse throughout the intron, while only those sequences directly adjacent to exons 1 and 2 are conserved between human and chicken. The unique sequence of pMbm-2 was located d irectly adjacent to exon 2, suggesting that it arose as a product of a lternative transcription initiation within intron 1. RNAase protection analysis was used to map a cluster of transcription start sites at th e 5' end of exon 2. Levels of messages utilizing these start sites are proportional to those arising from the primary promoter. Functional c haracterization of this region revealed that this region can function as a promoter. Deletion studies have revealed the presence of negative and positive regulatory elements within this region which are utilize d with different efficiencies in different cell lines. These studies s uggest that cis or trans factors acting in this region may serve a dua l function in both attenuation and transcription initiation.