Domains of U4 and U6 snRNAs required for snRNP assembly and splicing complementation in Xenopus oocytes

Structure‐function relationships in the vertebrate U4‐U6 snRNP have been analysed by assaying the ability of mutant RNAs to form U4‐U6 snRNPs and to function in splicing complementation in Xenopus oocytes. The mutants define three categories of domain within the RNAs. First, domains which are not essential for splicing. These include regions of U6 which have previously been implicated in the capping and transport to the nucleus of U6 RNA as well as, less surprisingly, regions of U4 and U6 which have been poorly conserved in evolution. Second, domains whose mutation reduces U4‐U6 snRNP assembly or stability. This group includes mutations in both the proposed U4‐U6 interaction domain, and also, in the case of U6, in a highly conserve sequence flanking stem I of the interaction domain. These mutants are all defective in splicing. Third, regions not required for U4‐U6 assembly, but required for splicing complementation. This category defines domains which are likely to be required for specific contacts with other components of the splicing machinery. Combinations of mutants in the U4 and U6 interaction domain are used to show that there are not only requirements for base complementarity but also for specific sequences in these regions.