The intramolecular stem-loop structure of U6 snRNA can functionally replace the U6atac snRNA stem-loop

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Abstract
The U6 spliceosomal snRNA forms an intramolecular stem-loop structure during spliceosome assembly that is required for splicing and is proposed to be at or near the catalytic center of the spliceosome. U6atac snRNA, the analog of U6 snRNA used in the U12-dependent splicing of the minor class of spliceosomal introns, contains a similar stem-loop whose structure but not sequence is conserved between humans and plants. To determine if the U6 and U6atac stem-loops are functionally analogous, the stem-loops from human and budding yeast U6 snRNAs were substituted for the U6atac snRNA structure and tested in an in vivo genetic suppression assay. Both chimeric U6/U6atac snRNA constructs were active for splicing in vivo. In contrast, several mutations of the native U6atac stem-loop that either delete putatively unpaired residues or disrupt the putative stem regions were inactive for splicing. Compensatory mutations that are expected to restore base pairing within the stem regions restored splicing activity. However, other mutants that retained base pairing potential were inactive, suggesting that functional groups within the stem regions may contribute to function. These results show that the U6atac snRNA stem-loop structure is required for in vivo splicing within the U12-dependent spliceosome and that its role is likely to be similar to that of the U6 snRNA intramolecular stem-loop.