= Small nuclear RNA
A <small nuclear RNA> is a member of a class of functional <RNAs> associated with the <cell nucleus>, many of which participate in nuclear <RNA splicing>. Their folded RNA and interactions contribute directly to processing other RNAs, rather than serving as a template for a <protein>. In a <small nuclear ribonucleoprotein>, the RNA is assembled with proteins; the RNA molecule and the complete particle are distinct objects.
<U1 small nuclear RNA> supplies a recognition example. Its complementary pairing with precursor-RNA <splice donor sites> was tested by introducing altered splice-site and U1 sequences together. A 1986 <compensatory base change> experiment rescued one tested defect efficiently but another only weakly.<A compensatory base change in U1 snRNA suppresses a 5′ splice site mutation by Y. Zhuang and A. M. Weiner>[Abstract, alternative donor-site perturbations and rescue]{ref} This ties sequence recognition to function while showing that restored complementarity alone need not restore every step of splicing.
<U6 small nuclear RNA> illustrates a different role. A 2013 study replaced selected RNA phosphate oxygens with sulfur and tested metal rescue in assembled yeast <spliceosomes>. The results identified U6 ligands positioning catalytic metals during both splicing reactions.<RNA catalyses nuclear pre-mRNA splicing by Sebastian M. Fica and others>[Page 229, Abstract; Figures 1–3, ligand substitution and metal-rescue comparisons]{ref} The evidence concerns RNA-mediated catalysis within a complex machine, not a conclusion that its proteins are dispensable.
Thus recognizing a transcript as a small nuclear RNA is a functional classification, not merely observing that it is short and lies in a nucleus. Its molecular partners, target interactions and perturbation response distinguish recognition, assembly and chemical catalysis.
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