IN-VITRO STUDY OF PROCESSING OF THE INTRON-ENCODED U16 SMALL NUCLEOLAR RNA IN XENOPUS-LAEVIS

It was recently shown that a new class of small nuclear RNAs is encode d in introns of protein-coding genes and that they originate by proces sing of the pre-mRNA in which they are contained. Little is known abou t the mechanism and the factors involved in this new type of processin g. The L1 ribosomal protein gene of Xenopus laevis is a well-suited sy stem for studying this phenomenon: several different introns encode fo r two small nucleolar RNAs (snoRNAs; U16 and U18). In this paper, we a nalyzed the in vitro processing of these snoRNAs and showed that both are released from the pre-mRNA by a common mechanism: endonucleolytic cleavages convert the pre-mRNA into a precursor snoRNA with 5' and 3' trailer sequences. Subsequently, trimming converts the pre-snoRNAs int o mature molecules. Oocyte and HeLa nuclear extracts are able to proce ss X. laevis and human substrates in a similar manner, indicating that the processing of this class of snoRNAs relies on a common and evolut ionarily conserved mechanism. In addition, we found that the cleavage activity is strongly enhanced in the presence of Mn2+ ions.