Strong dependence between functional domains in a dual-function snoRNA infers coupling of rRNA processing and modification events.

Most small nucleolar RNAs (snoRNAs) guide rRNA nucleotide modifications, some participate in pre-rRNA cleavages, and a few have both functions. These activities involve direct base-pairing of the snoRNA with pre-rRNA using different domains. It is not known if the modification and processing functions occur independently or in a coordinated manner. We address this question by mutational analysis of a yeast box H/ACA snoRNA that mediates both processing and modification. This snoRNA (snR10) contains canonical 5'- and 3'-hairpin structures with a guide domain for pseudouridylation in the 3' hairpin. Our functional mapping results show that: (i) processing requires the 5' hairpin exclusively, in particular a 7-nt element; (ii) loss of the 3' hairpin or pseudouridine does not affect rRNA processing; (iii) a single nucleotide insertion in the guide domain shifts modification to an adjacent uridine in rRNA, and severely impairs both processing and cell growth; and (iv) the deleterious effects of the insertion mutation depend on the presence of the processing element in the 5' hairpin, but not modification of the novel site. Together, the results suggest that the snoRNA hairpins function in a coordinated manner and that their interactions with pre-rRNA could be coupled.