= U2 small nuclear RNA
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\b[U2 small nuclear RNA] is a <small nuclear RNA> that helps recognize the <RNA splicing branch point> and contributes to the active RNA arrangement of the <spliceosome>. It acts as part of an RNA–protein particle. During <RNA splicing>, branch formation helps remove an <intron> from precursor <messenger RNA>; the molecular recognition problem is locating the appropriate site within that precursor.
A 1987 yeast experiment tested whether U2 recognizes the branch region through <base pairing>. Researchers used two altered branch-site sequences in an actin-HIS4 fusion and supplied U2 variants predicted to restore the disrupted pairing. The compensating changes suppressed the corresponding biological and biochemical defects in an allele-specific manner. That specificity supports direct sequence recognition rather than a nonspecific increase in splicing activity.<Recognition of the TACTAAC box during mRNA splicing in yeast involves base pairing to the U2-like snRNA by Roy Parker and others>[Abstract, two branch-site mutants and compensating U2 variants.]{ref}
A simplified pairing example explains the logic without reproducing an experimental sequence. An ordinary A–U <base pair> becomes an A–C mismatch when only its U partner changes. Changing the opposite A to G can restore a G–C pair. Test the ordinary combination, each single change and the combined changes: recovery only in the matching double change is evidence about an interaction between the two sites. Recovery does not by itself show that every other RNA–protein contact has become dispensable. This is the logic of a <compensatory base change> experiment.
Recognition is not U2's only contribution. U2 also pairs with <U6 small nuclear RNA> in the catalytic RNA core.<RNA catalyses nuclear pre-mRNA splicing by Sebastian M. Fica and others>[p.229, Figure1: U2/U6 helix Ib and the U6 catalytic region.]{ref} Following both interactions connects branch-site selection to assembly of the reaction machinery, while keeping sequence recognition distinct from the chemical steps of splicing.
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