Distinct properties of c-myc transcriptional elongation are revealed in Xenopus oocytes and mammalian cells and by template titration, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), and promoter mutagenesis.

A block to c-myc transcription elongation has been observed in Xenopus oocytes and mammalian cells. Here, we show that the distribution of RNA polymerase II transcription complexes in the c-myc promoter proximal region in Xenopus oocytes is different from that observed previously in mammalian cells. Thus, there are major differences in the c-myc elongation block observed in the two systems. In addition, as first reported for a Xenopus tubulin gene (K. M. Middleton and G. T. Morgan, Mol. Cell. Biol. 10:727-735, 1990). c-myc template titration experiments reveal the existence of two classes of RNA polymerase II transcription complexes in oocytes: one (at low template concentration) that is capable of reading through downstream sites of premature termination, and another (high template concentration) that does not. We show that these classes of polymerases are distinct from those previously identified by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), which distinguishes transcription complexes on the basis of transcribed distance, rather than on the basis of differential elongation through sites of premature termination. We also show that mutations that affect the efficiency of initiation of transcription from the c-myc P2 promoter can influence premature termination by at least two mechanisms: TATA box mutations function by the titration effect (decrease in transcription initiation results in a relative decrease in premature termination), while an upstream activator (E2F) site functions by contributing to the assembly of polymerase complexes competent to traverse the downstream sites of premature termination.