PROCESSING OF THE INTRON-ENCODED U16 AND U18 SNORNAS - THE CONSERVED C-BOX AND D-BOX CONTROL BOTH THE PROCESSING REACTION AND THE STABILITYOF THE MATURE SNORNA

A novel class of small nucleolar RNAs (snoRNAs), encoded in introns of protein coding genes and originating from processing of their precurs or molecules, has recently been described. The L1 ribosomal protein (r -protein) gene of Xenopus laevis and its human homologue contain two s noRNAs, U16 and U18. It has been shown that these snoRNAs are excised from their intron precursors by endonucleolytic cleavage and that thei r processing is alternative to splicing, Two sequences, internal to th e snoRNA coding region, have been identified as indispensable for proc essing the conserved boxes C and D. Competition experiments have shown that these sequences interact with diffusible factors which can bind both the pre-mRNA and the mature U16 snoRNA. Fibrillarin, which is kno wn to associate with complexes formed on C and D boxes of other snoRNA s, is found in association with mature U16 RNA, as well as with its pr ecursor molecules. This fact suggests that the complex formed on the p re-mRNA remains bound to U16 throughout all the processing steps. We a lso show that the complex formed on the C and D boxes is necessary to stabilize mature snoRNA.