The U1 snRNP recognizes and binds to the sequence of the 5'-splice site of an intron in a strand of pre-mRNA. Experimentation has demonstrated that the binding of U1 snRNA to the 5'-splice site is required, but not sufficient, to begin spliceosome assembly. 
There are significant differences in sequence and secondary structure between metazoan and yeast U1 snRNAs, the latter being much longer (568 nucleotides as compared to 164 nucleotides in humans). Nevertheless, secondary structure predictions suggest that all U1 snRNAs share a 'common core' consisting of helices I, II, the proximal region of III, and IV. This family does not contain the larger yeast sequences.
A non-canonical role for U1 snRNP has recently been described in the regulation of alternative polyA site selection It is proposed that increased transcription rates "sponge" U1 snRNP, decreasing its availability. This model is supported experimentally, as reducing U1 snRNP levels with antisensemorpholino oligonucleotides led to a dose-dependent shift of polyA usage to generate shorter mRNA transcripts.
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