Three Small Nucleolar RNAs Identified from the Spliced Leader-Associated RNA Locus in Kinetoplastid Protozoans

ABSTRACT

First characterized in Trypanosoma brucei, the spliced leader-associated (SLA) RNA gene locus has now been isolated from the kinetoplastids Leishmania tarentolae and Trypanosoma cruzi. In addition to the T. brucei SLA RNA, bothL. tarentolae and T. cruzi SLA RNA repeat units also yield RNAs of 75 or 76 nucleotides (nt), 92 or 94 nt, and ∼450 or ∼350 nt, respectively, each with significant sequence identity to transcripts previously described from the T. brucei SLA RNA locus. Cell fractionation studies localize the three additional RNAs to the nucleolus; the presence of box C/D-like elements in two of the transcripts suggests that they are members of a class of small nucleolar RNAs (snoRNAs) that guide modification and cleavage of rRNAs. Candidate rRNA-snoRNA interactions can be found for one domain in each of the C/D element-containing RNAs. The putative target site for the 75/76-nt RNA is a highly conserved portion of the small subunit rRNA that contains 2′-O-ribose methylation at a conserved position (Gm1830) in L. tarentolae and in vertebrates. The 92/94-nt RNA has the potential to form base pairs near a conserved methylation site in the large subunit rRNA, which corresponds to position Gm4141 of small rRNA 2 in T. brucei. These data suggest that trypanosomatids do not obey the general 5-bp rule for snoRNA-mediated methylation.

ACKNOWLEDGMENTS

We thank Shula Michaeli and Jan Dungan for helpful discussions and Jörg Hoheisel, Jaime M. Santana, Marc Christensen, and Roberto Hernández for providing invaluable reagents and information for this study. We also thank Aparche Yang for contributions to the project.

This work was funded by NIH grants AI34536 to D.A.C., AI21975 to N.A., and AI34093 to T.H. The Microbial Pathogenesis Training Grant, 5-T32-AI-07323, supported trainees N.R.S. and B.K.Y.