CHARACTERIZATION OF A NOVEL TRYPANOSOMATID SMALL NUCLEOLAR RNA

Trypanosomes possess unique RNA processing mechanisms including trans- splicing of pre-mRNA and RNA editing of mitochondrial transcripts, The previous finding of a trimethylguanosine (TMG) capped U3 homologue in trypanosomes suggests that rRNA processing may be related to the proc essing in other eukaryotes, In this study, we describe the first trypa nosomatid snoRNA that belongs to the snoRNAs that were shown to guide ribose methylation of rRNA, The RNA, identified in the monogenetic try panosomatid Leptomonas collosoma, was termed snoRNA-2 and is encoded b y a multi-copy gene. SnoRNA-2 is 85 nt long, it lacks a 5' cap and pos sesses the C and D boxes characteristic to all snoRNAs that bind fibri llarin. Computer analysis indicates a potential for base-pairing betwe en snoRNA-2 and 5.8S rRNA, and 18S rRNA. The putative interaction doma ins obey the rules suggested for the interaction of guide snoRNA with its rRNA target for directing ribose methylation on the rRNA, However, mapping the methylated sites on the 5.8S rRNA and 18S rRNA indicates that the expected site on the 5.8S is methylated, whereas the site on the 18S is not, The proposed interaction with 5.8S rRNA is further sup ported by the presence of psoralen cross-link sites on snoRNA-2, GenBa nk search suggests that snoRNA-2 is not related to any published snoRN As, Because of the early divergence of the Trypanosomatidae from the e ukaryotic lineage, the presence of a methylating snoRNA that is encode d by a multi-copy gene suggests that methylating snoRNAs may have evol ved in evolution from self-transcribed genes.