REPEATED CT ELEMENTS BOUND BY ZINC-FINGER PROTEINS CONTROL THE ABSOLUTE AND RELATIVE ACTIVITIES OF THE 2 PRINCIPAL HUMAN C-MYC PROMOTERS

Transcription of the human proto-oncogene c-myc is governed by two tan dem principal promoters, termed P1 and P2. In general, the downstream promoter, P2, is predominant, which is in contrast to the promoter occ lusion phenomenon usually observed in genes containing tandem promoter s. A shift in human c-myc promoter usage has been observed in some tum or cells and in certain physiological conditions. However, the mechani sms that regulate promoter usage are not well understood. The present studies identify regulators which are required to promote transcriptio n from both human c-myc promoters, P1 and P2, and have a role in deter mining their relative activities in vivo. A novel regulatory region lo cated 101 bp upstream of P1 was characterized and contains five tandem repeats of the consensus sequence CCCTCCCC (CT element). The integrit y of the region containing all five elements is required to promote tr anscription from P1 and for maximal activity from P2 in vivo. A single copy of this same element, designated CT-I2, also appears in an inver ted orientation 53 bp upstream of the P2 transcription start site. Thi s element has an inhibitory effect on P1 transcription and is required for P2 transcription. The transcription factor Sp1 was identified as the factor that binds specifically to the tandem CT elements upstream of P1 and to the CT-I2 element upstream of P2. In addition, the recent ly cloned zinc finger protein ZF87, or MAZ, was also able to bind thes e same elements in vitro. The five tandem CT elements can be functiona lly replaced by a heterologous enhancer that only in the absence of CT -I2 reverses the promoter usage, similar to what is observed in the tr anslocated c-myc allele of Burkitt's lymphoma cells.