STRUCTURE OF THE LEPTOMONAS SEYMOURI TRANS-SPLICEOSOMAL U2 SNRNA-ENCODING GENE - POTENTIAL U2-U6 SNRNA INTERACTIONS CONFORM TO THE CIS-SPLICING COUNTERPART

We have characterized the U2 small nuclear RNA (snRNA)-encoding gene f rom the monogenetic trypanosomatid, Leptomonas seymouri (Ls), to begin to identify the RNA-RNA interactions that direct trans-splicing in ki netoplastid protozoa. The U2 gene, which is single copy in this organi sm, was isolated and sequenced. Although the Ls U2 snRNA contains many of the sequence and secondary structure elements that are conserved a mong the U2 snRNAs of cis-splicing organisms, it lacks the stem-loop I II region and the intron branch point-recognition region, as do other trypanosomatid U2 snRNAs. A transcriptional promoter element within th e Trypanosoma brucei U2 gene [Fantoni et al., Mel. Cell. Biol. 14 (199 4) 2021-2028] is conserved in the homologous Ls gene. A crucial step i n cis-splicing reactions involves specific base-pairing interactions b etween the U2 and U6 snRNAs. We show here that in trypanosomatids, whe re no cis-splicing occurs, these same interactions are possible. This highlights key similarities between the two RNA processing events.