Precise identification of cleavage sites involved in the unusual processing of trypanosome ribosomal RNA.

The large subunit ribosomal RNA (LSRNA) of Trypanosoma brucei is unusual in being cleaved at multiple sites to yield six stable fragments of RNA. We report here the complete nucleotide sequence of two regions of the ribosomal DNA repeat unit. The first sequence includes all of the processing sites involved in the generation of one of the small LSRNA fragments. The second region encodes the trypanosome 5.8 S RNA. By RNA sequencing and S1 nuclease mapping, we have identified the processing sites involved in the generation of both of these small RNAs. On the basis of predicted secondary structure models, we infer that all the cleavages apparently occur near the junction of single- and double-stranded regions. The sites involved in the novel LSRNA processing show a clear symmetry with respect to a conserved region of ten base-pairs. No such signals are evident for the processing sites that generate the 5.8 S RNA.