Mouse and frog violate the paradigm of species-specific transcription of ribosomal RNA genes.

Transcription of ribosomal RNA genes by RNA polymerase I is generally accepted as being highly species specific, a conclusion based on numerous reports that rRNA genes of one species are not transcribed by factors of even closely related species. It thus was striking to find that cloned rDNA from the frog Xenopus laevis is specifically transcribed in extracts prepared from mouse cells. The data in this paper demonstrate that this heterologous transcription is due to a normal initiation process and not to a fortuitous event. Transcription of Xenopus rDNA in the mouse cell extract is directed by the same large promoter (residue-141 to +6) that is utilized to promote the synthesis of frog rRNA in homologous Xenopus systems. Moreover, the same factors of the mouse cell extract that transcribe the homologous mouse rDNA also catalyze transcription from the X. laevis rDNA promoter. We conclude that polymerase I transcriptional machinery does not evolve as rapidly as prior studies would suggest.