= Splice donor site
A \b[splice donor site] is the <splice site> at the $5'$ end of an <intron> in a precursor <RNA>. It marks the boundary between the upstream <exon> and the <intron>. Its name is oriented along the precursor <RNA>, not along whichever direction a <DNA> reference sequence happens to be displayed.
In the toy precursor `A|intron|B`, with letters representing whole <exons>, the first boundary is the <splice donor site> and the second is the <splice acceptor site>. Removing the <intron> and joining A to B gives the mature <splice junction>. A change in donor choice can change the end of A even when acceptor choice remains fixed; this is one route to <alternative splicing>.
Recognition involves sequence and molecular context. The <RNA> component of <U1 snRNP> can base-pair with the donor region. A particularly informative experiment changed a donor sequence and then changed U1 to restore complementarity. Rescue of one donor mutation supported direct pairing; weak rescue of another showed that complementarity alone did not guarantee productive <RNA splicing>.<A compensatory base change in U1 snRNA suppresses a 5′ splice site mutation by Y. Zhuang and A. M. Weiner>[Abstract, compensatory changes at E1A donor positions+5 and+3, co-transfection and unequal suppression]{ref}
This is a useful causal test: perturbing either partner can disrupt an interaction, while a compensating change in the other partner can distinguish a specific pairing model from a generic loss of function. A candidate sequence bound by <U1 snRNP> still has to participate in the rest of the <spliceosome>'s reaction.
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