THE BLOCK TO TRANSCRIPTION ELONGATION IS PROMOTER DEPENDENT IN NORMAL AND BURKITTS-LYMPHOMA C-MYC ALLELES

Aberrant c-myc expression patterns occur in human Burkitt's lymphoma cells, which consistently exhibit c-myc chromosomal translocations, mutations within and flanking the translocated allele, a loss of the block to transcription elongation in exon 1, and a promoter shift to use of the upstream P1 promoter. To define the mechanism responsible for the loss of transcription elongation blockage and resulting c-myc deregulation in Burkitt's lymphoma, we analyzed transcription patterns after transfer of normal and Burkitt's lymphoma c-myc alleles into murine cells and Xenopus oocyte germinal vesicles. We have determined that although the mutations within and surrounding several Burkitt's lymphoma c-myc alleles are not sufficient, in themselves, to abrogate the transcription elongation block, transcription initiation from the P2 promoter may be necessary to obtain the block to transcription elongation. To test directly the role of c-myc promoters in programming transcription elongation blockage, we analyzed transcription patterns from in vitro mutagenized c-myc genes containing deletions of either the P1 or P2 promoter. These data confirm that P1-initiated c-myc transcripts do not terminate at discrete sites near the 3′ end of exon 1, whereas P2-initiated transcripts either terminate or read through the transcription block signals. Therefore, overexpression and/or constitutive expression from the c-myc P1 promoter may contribute to increased readthrough transcription in Burkitt's lymphoma cells and, hence, to aberrant expression patterns or levels of c-myc steady-state transcripts. In addition, the ability of normal cells to modulate c-myc P2-initiated transcription to either read through or to block elongation provides a fine control mechanism over c-myc steady-state RNA levels.