= U6 small nuclear RNA
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\b[U6 small nuclear RNA] is a functional <small nuclear RNA> of the <spliceosome>. It helps construct the catalytic RNA arrangement that removes an <intron> from a precursor <messenger RNA> and joins the flanking <exons>. Pairing with <U2 small nuclear RNA> and folding within U6 organize part of this active core. U6 is a functional molecular component rather than a message translated into a <protein>.
The <U6 metal rescue experiment> tests a chemical role beyond simple binding. In assembled yeast <spliceosomes>, investigators replaced selected phosphate oxygens of U6 with <sulfur> and measured splicing with different divalent metal ions. Rescue patterns identified RNA ligands positioning catalytic metals during both splicing reactions. The tested machinery still contained <proteins>; this was evidence for RNA-mediated catalysis within the <spliceosome>.<RNA catalyses nuclear pre-mRNA splicing by Sebastian M. Fica and others>[pp. 229–231, Figures 1–3: U2/U6 structure, individual sulfur substitutions, branching and exon-ligation metal-rescue assays.]{ref}
Why compare more than the damaged RNA in one metal? Consider an invented control table with activities relative to a reference: ordinary RNA gives $1.0$ in metal A and $0.8$ in metal B; altered RNA gives $0.05$ in A and $0.60$ in B. B decreases the ordinary-RNA activity, yet raises the altered-RNA activity twelvefold. That interaction is more informative than saying B simply makes every reaction faster. These numbers illustrate the comparison; they are not measurements from the experiment.
Even this pattern needs structural and chemical controls: an alteration could change folding, assembly or the measured reaction. Separating catalytic activation from the final chemical assay helps distinguish a direct chemical defect from failure to assemble the machine. Connecting RNA chemistry to reaction output is what makes U6 an instructive example of an RNA active core within a larger RNA–protein complex.
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