Species Specificity in Ribosome Biogenesis: a Nonconserved Phosphoprotein Is Required for Formation of the Large Ribosomal Subunit in Trypanosoma brucei

In the protozoan parasite Trypanosoma brucei, the large rRNA, which is a single 3.4- to 5-kb species in most organisms, is further processed to form six distinct RNAs, two larger than 1 kb (LSU1 and LSU2) and four smaller than 220 bp. The small rRNA SR1 separates the two large RNAs, while the remaining small RNAs are clustered at the 3' end of the precursor rRNA. One would predict that T. brucei possesses specific components to carry out these added processing events. We show here that the trypanosomatid-specific nucleolar phosphoprotein NOPP44/46 is involved in this further processing. Cells depleted of NOPP44/46 by RNA interference had a severe growth defect and demonstrated a defect in large-ribosomal-subunit biogenesis. Concurrent with this defect, a significant decrease in processing intermediates, particularly for SR1, was seen. In addition, we saw an accumulation of aberrant processing intermediates caused by cleavage within either LSU1 or LSU2. Though it is required for large-subunit biogenesis, we show that NOPP44/46 is not incorporated into the nascent particle. Thus, NOPP44/46 is an unusual protein in that it is both nonconserved and required for ribosome biogenesis.