Sequences in the human c-myc P2 promoter affect the elongation and premature termination of transcripts initiated from the upstream P1 promoter.

A conditional block to transcription elongation provides one mechanism for controlling the steady-state levels of c-myc RNA in mammalian cells. Although prematurely terminated c-myc RNAs are not detectable in mammalian cells, truncated c-myc RNAs with 3' ends that map near the end of the first exon are transcribed from human c-myc templates injected into Xenopus oocytes germinal vesicles. A series of linker scanner and deletion mutants within the c-myc P2 promoter was tested in the Xenopus oocyte injection assay to determine the potential contribution of promoter elements to the elongation or premature termination of c-myc transcription. Although this analysis failed to identify sequences in the P2 promoter that significantly affect the elongation or termination of P2-initiated transcripts, our results suggest that sequences within the P2 promoter contribute to the premature termination of transcripts initiated at the upstream P1 promoter. A subset of these sequences is essential for the efficient elongation of P1-initiated transcripts through intrinsic sites of termination at the end of exon 1. These sequences affect P1 elongation when they are downstream of the site of initiation, and we hypothesize that they may be analogous to a class of prokaryotic elements required for antitermination.