= RNA polymerase I
{c}
<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. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 6, The Nucleolus Is a Ribosome-Producing Factory]{ref}
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. <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]{ref}
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