THE U6 SNRNA-ENCODING GENE OF THE MONOGENETIC TRYPANOSOMATID LEPTOMONAS-COLLOSOMA

The U6 snRNA (U6) is the most conserved small nuclear RNA (snRNA) and apparently plays a central role in catalysis of the cis-splicing react ion. In trans-splicing, U6 may have an additional function. In the nem atode trans-splicing system, a direct interaction between the U6 and s pliced leader (SL) RNAs has been demonstrated, suggesting that U6 may serve as a bridge between the SL RNA and the acceptor pre-mRNA. To exa mine possible phylogenetic conservation of trypanosomatid U6 sequences that may interact with spliceosomal RNAs, we have cloned and sequence d the U6 gene from the monogenetic trypanosomatid Leptomonas collosoma (Lc). The Le U6 deviates from the Trypanosoma brucei (Tb) RNA only in four positions located in the 5' stem-loop and the central domains. A s in Tb, U6 is a single-copy gene and two tRNA genes, tRNA(Gln) and tR NA(Ile), are found upstream to the gene. The tRNAs are differentially expressed; tRNA(Gln) is transcribed in the opposite direction to U6, w hereas tRNA(Ile) is not transcribed. Possible base-pairing between U6 and the U2 and SL RNAs, similar to the interactions that take place in the nematode trans-splicing system, are proposed.