The 5' leads to 3' polarity of the Xenopus Ribosomal RNA precursor molecule.

Evidence is presented that in Xenopus laevis the 18S rRNA sequence is proximal to the 5' end of the rRNA precursor molecule and the 28S secquence is proximal to the 3' end. This assignment was made by digesting amplified ribosomal gene transcription complexes with EcoRI restriction endonuclease and spreading the cleaved transcription complexes for electron microscopy. From the known location of the EcoRI cutting sites within the transcribed region and the length of nascent chains on the cleaved transcription complexes, it was possible to make an unambiguous assignment of polarity.