Identification of the First Trypanosome H/ACA RNA that Guides Pseudouridine Formation on rRNA

In trypanosomes small nucleolar RNA (snoKNA) genes are clustered, and the clusters encode for either single or multiple RNAs. We previously reported on a genomic locus in Leptomonas collosoma that encodes for multiple C/D snoRNAs whose expression is regulated at the processing level (Xu, Y., Liu, L., Lopez-Estraño, C., and Michaeli, S. (2001) J. Biol. Chem. 276, 14289-14298). In this study we have characterized, in the same genomic locus, the first trypanosome H/ACA RNA, which we termed hi. Having a length of 69 nucleotides, hi has the potential to guide pseudouridylation on 28 S rRNA. The hi is processed from a long polycistronic transcript that carries both the C/D and hi snoRNAs. The h1/rRNA duplex obeys the rules for guiding pseudouridylation. Mapping of the pseudouridine site indicated that the predicted U is indeed modified. However, in contrast to all H/ACA RNAs, hi consists of a single hairpin structure and is the shortest H/ACA RNA described so far.