PROCESSING OF THE YEAST PRE-RIBOSOMAL-RNA AT SITES A(2) AND A(3) IS LINKED

Cleavage of the yeast pre-rRNA at site A(2) in internal transcribed sp acer 1 (ITS1) requires multiple snoRNP species, whereas cleavage at si te A(3), located 72 nt 3' in ITS1, requires RNase MRP. Analyses of mut ations in the pre-rRNA have revealed an unexpected link between proces sing at A(2) and A(3). Small substitution mutations in the 3' flanking sequence at A(2) inhibit processing at site A(3), whereas a small del etion at A(3) has been shown to delay processing at site A(2). Moreove r, the combination of mutations in cis at both A(2) and A(3) leads to the synthesis of pre-rRNA species with 5' ends within the mature 18S r RNA sequence, at sites between +482 and +496. The simultaneous interfe rence with an snoRNP processing complex at site A(2) and an RNase MRP complex at site A(3) may activate a pre-rRNA breakdown pathway. The sa me aberrant pre-rRNA species are observed in strains with mutations in the RNA component of RNase MRP, consistent with interactions between the processing complexes. Furthermore, genetic depletion of the snoRNA , snR30, has been shown to affect the coupling between cleavage by RNa se MRP and subsequent exonuclease digestion. We conclude that an snoRN P-dependent processing complex that is required for A(2) cleavage and that recognizes the 3' flanking sequence at A(2), interacts with the R Nase MRP complex bound to the pre-rRNA around site A(3).