Localization of small nuclear polymerase I RNA sequences at the 5' end of the human rDNA transcription unit.

A human ribosomal DNA clone isolated from a genomic library was used to localize the DNA sequences coding for HeLa cell small nuclear polymerase I RNA (snPI RNA). By using a subcloned 1.2-kb EcoRI-SalI fragment, including the initiation region of the 45S rRNA transcription unit, it was shown that the snPI RNA sequences are located within the first 600 nucleotides of the 5' end of the external transcribed spacer. Strand-specific hybridization following exonuclease III digestion of the plasmid containing the 1.2-kb Eco-Sal fragment demonstrated that the snPI RNA molecules and the 45S pre-rRNA are transcribed from the same coding strand. A detailed mapping of individual snPI RNA molecules showed that most of these small RNA species span the putative early processing site at position 415 of the external transcribed spacer of the human rRNA precursor.