In vivo UV cross-linking of U snRNAs that participate in trypanosome trans-splicing.

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Abstract
The maturation of mRNAs in Trypanosoma brucei involves a trans-splicing reaction whereby the 5' 39 nucleotides of a small RNA, called the spliced leader (SL) RNA, are joined with a pre-mRNA transcript. The trans-splicing reaction appears mechanistically similar to cis-splicing of nuclear pre-mRNAs, and homologs of the U2, U4, and U6 snRNAs are required for the process. In the work presented here, potential RNA-RNA interactions between the SL RNA and the U snRNAs of trypanosomes were examined by UV light induction of RNA-RNA cross-links in vivo. We detected cross-linkage between U2 and U6 RNAs and, as might be expected, between the trypanosome U4 and U6 RNAs. The latter contain extensive sequence complementarity and are thought to exist predominantly in a single RNP. We also detected an SL RNA species following in vivo UV treatment, which may represent either an intramolecular cross-link in the SL RNA or a cross-link formed between the SL RNA and an as yet unidentified small RNA. Mapping of the cross-link position between U2 and U6 RNAs is consistent with base-pairing between the 5' domain of U2 and the 3' end of U6 RNA. These results reveal the existence, in vivo, of cognate RNA-RNA interactions in the RNA homologs that participate in trans-splicing in trypanosomes and cis-splicing in other eukaryotes.