A protein binding site from the murine c-myc promoter contributes to transcriptional block

Recent studies have revealed that expression of several eukaryotic genes can be regulated at the level of transcription elongation. As a first step to elucidate the mechanism by which transcription elongation is modulated, several groups have identified sequences necessary for transcriptional block within the c-myc gene. These studies indicated that transcriptional block depends not only on sequences surrounding the sites of block, but also on sequences within the promoter: some deletions within the c-myc promoter eliminated transcriptional block and, with chimeric constructs, transcriptional block was observed when some heterologous promoters but not others were fused to the c-myc termination region. Using a chimeric construct containing the H-2Kb major histocompatibility class gene promoter linked to the c-myc first exon, we show that transcriptional block is increased by the addition of a 25 bp DNA sequence from the c-myc promoter. Similar results are obtained whether this sequence is inserted upstream or downstream of the transcription initiation site. We further show that nuclear factors interact with this sequence in vitro. Interestingly, when a mutated version of this sequence was tested, we observed decreased nuclear factor binding in vitro as well as reduced transcriptional block in nuclear run-on transcription assays. These results suggest that interactions of protein factors with specific nucleotide sequences near the transcription initiation site can affect elongation of transcription at sites located further downstream.