RNA polymerase I is a eukaryotic RNA polymerase specialized for synthesis of the large precursor from which the major ribosomal RNAs, including 18S, 5.8S and 28S in mammals, are processed. It acts in the nucleolus. The separate 5S ribosomal RNA is normally transcribed by RNA polymerase III, so not all ribosomal RNA has the same polymerase origin. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 6, The Nucleolus Is a Ribosome-Producing Factory
A yeast experiment disrupted the essential polymerase-I subunit RPA43. Driving ribosomal-RNA synthesis through a different polymerase could rescue the lethal defect, linking the essential requirement to RNA production. The rescue distinguishes the normal route from an experimentally engineered substitute, rather than implying that the alternative route is standard. [ref]
Gene RPA43 in Saccharomyces cerevisiae Encodes an Essential Subunit of RNA Polymerase I by Pierre Thuriaux and others. 1995, abstract: disruption and alternative-polymerase rescue
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