= U1 small nuclear RNA
{c}
= U1 snRNA
{c}
{synonym}
\b[U1 small nuclear RNA], or <U1 snRNA>, is the <small nuclear RNA> component of <U1 snRNP>. The <RNA> and the assembled <ribonucleoprotein> are distinct objects: the particle also contains <proteins>. U1's $5'$ region recognizes many <splice donor sites> by complementary pairing during spliceosomal <RNA splicing>.<A compensatory base change in U1 snRNA suppresses a 5′ splice site mutation by Y. Zhuang and A. M. Weiner>[Abstract, recognition of precursor-RNA donor sites by the U1 5′ end]{ref}
A simple base-pair model illustrates a compensatory test. Suppose a donor base A pairs with a U in U1. Changing the donor A to C creates a C–U mismatch. Changing the corresponding U1 base to G restores a C–G pair. This is a hypothetical pair chosen to explain the experiment's logic, rather than the exact sequence of a published mutant.
In the original test, mutations at two alternative donor sites were paired with compensating changes in human U1. One mutation at donor position+5 was efficiently suppressed, whereas a different mutation at position+3 was only weakly suppressed. The unequal rescue supported direct pairing while showing that pairing alone was insufficient for productive splicing.<A compensatory base change in U1 snRNA suppresses a 5′ splice site mutation by Y. Zhuang and A. M. Weiner>[Abstract, adenovirus E1A 12S/13S donor mutations and co-transfection experiments]{ref}
The comparison distinguishes molecular recognition from the entire processing pathway. Restoring complementarity tests a particular RNA–RNA contact; it does not establish that every later event in <spliceosome> assembly and catalysis has also been restored. Similarly, the presence of <U1 snRNA> alone is not equivalent to assembling a complete active <U1 snRNP>.
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