U1-like snRNAs lacking complementarity to canonical 5′ splice sites

We have detected a surprising heterogeneity among human spliceosomal U1 small nuclear RNA (snRNA). Most interestingly, we have identified three U1 snRNA variants that lack complementarity to the canonical 5′ splice site (5′SS) GU dinucleotide. Furthermore, we have observed heterogeneity among the identified variant U1 snRNA genes caused by single nucleotide polymorphism (SNP). The identified snRNAs were ubiquitously expressed in a variety of human tissues representing different stages of development and displayed features of functional spliceosomal snRNAs, i.e., trimethylated cap structures, association with Sm proteins and presence in nuclear RNA–protein complexes. The unanticipated heterogeneity among spliceosomal snRNAs could contribute to the complexity of vertebrates by expanding the coding capacity of their genomes.