Strong transcriptional activation of translocated c-myc genes occurs without a strong nearby enhancer or promoter

We have studied the transcriptional activation of translocated c-myc genes in murine plasmacytomas in which the translocation juncture occurs within the first intron of c-myc and juxtaposes c-myc with the immunoglobulin C alpha gene segment. It has been widely suggested that a novel transcriptional enhancer element located near the C alpha gene segment might activate the translocated c-myc gene. We have carried out an extensive search for such an element and find no significant transcriptional enhancer activity in a 22 kb region encompassing the translocation junction, C alpha gene segment and regions 3' of C alpha. We also find that the cryptic promoter region of the translocated c-myc gene is a very weak promoter of transcription. Despite this evidence against the presence of strong transcriptional regulatory elements, the translocated c-myc gene locus is transcribed at high rates that are 25-greater than 100% of that measured for the highly active immunoglobulin genes in murine plasmacytomas. These data suggest the presence of a novel type of strong activator of transcription in the murine heavy chain locus.