The origin of the rRNA precursor from Xenopus borealis, analysed in vivo and in vitro.

We have determined the origin of the major transcript of Xenopus borealis rDNA by the use of an SI nuclease protection assay. The DNA surrounding the origin of this transcript was sequenced, and the region upstream of the origin was shown to have strong sequence homology with that region from X.laevis rDNA. We have also demonstrated faithful transcription from this origin using cloned X.borealis rDNA in an extract derived from X. laevis culture cells. This in vitro transcription was insensitive to 100 micrograms/ml alpha-amanatin, suggesting that it was mediated by RNA polymerase 1.