Coexistence of a c-myc mRNA initiated in intron 1 with the normal c-myc mRNA and similar regulation of both transcripts in mammalian cells.

We previously described a c-myc mRNA initiated within intron 1 at a major start site designated P3. This mRNA was observed in tumorigenic murine cells with no detectable rearrangement of the c-myc gene. In the present study, we report that P3 mRNA also coexists with the P1/P2 c-myc transcript in non-tumorigenic murine cell lines and that a c-myc mRNA which appears to be an equivalent of the murine P3 transcript is found in various rat and human cells. These data suggest that P3 represents a promoter of the normal, non-rearranged mammalian c-myc gene. Furthermore, both P1/P2 and P3 mRNA levels are similarly regulated by cycloheximide and by dimethylsulfoxide (DMSO) in murine erythroleukemia Friend cells.