Base pairing with U6atac snRNA is required for 5′ splice site activation of U12-dependent introns in vivo

The minor U12-dependent class of eukaryotic nuclear pre-mRNA introns is spliced by a distinct spliceosomal mechanism that requires the function of U11, U12, U5, U4atac, and U6atac snRNAs. Previous work has shown that U11 snRNA plays a role;similar to U1 snRNA in the major class spliceosome by base pairing to the conserved 5' splice site sequence. Here we show that U6atac snRNA also base pairs to the 5' splice site in a manner analogous to that of U6 snRNA in the major class spliceosome. We show that splicing defective mutants of the 5' splice site can be activated for splicing in vivo by the coexpression of compensatory U6atac snRNA mutants. In some cases, maximal restoration of splicing required the coexpression of compensatory U11 snRNA mutants. The allelic specificity of mutant phenotype suppression is consistent with Watson-Crick base pairing between the pre-mRNA and the snRNAs. These results provide support for a model of the RNA-RNA interactions at the core of the U12-dependent spliceosome that is strikingly similar to that of the major class U2-dependent spliceosome.