Precursors to the U3 small nucleolar RNA lack small nucleolar RNP proteinsbut are stabilized by La binding

Almost all small eukaryotic RNAs are processed from transiently stabilized 3'-extended forms, A key question is how and why such intermediates are sta bilized and how they can then be processed to the mature RNA. Here,ve repor t that yeast U3 is also processed from a 3'-extended precursor, The major 3 '-extended farms of U3 (U3-3'I and -II) lack the cap trimethylation present in mature U3 and are not associated with small nucleolar RNP (snoRNP) prot eins that bind mature U3, i.e., Nop1p, Nop56p, and Nop58p, Depletion of Nop 58p leads to the loss of mature U3 but increases the level of U3-3'I and -I I, indicating a requirement for the snoRNP proteins for final maturation. P re-U3 is cleaved by the endonuclease Rnt1p, but U3-3'I and -II do not exten d to the Rnt1p cleavage sites. Rather, they terminate at poly(U) tracts, su ggesting that they might be bound by Lhp1p (the yeast homologue of La). Imm unoprecipitation of Lhp1p fused to Staphylococcus aureus protein A resulted in coprecipitation of both U3-3'I and -II. Deletion of LHP1, which is none ssential, led to the loss of U3-3'I and -II, We conclude that pre-U3 is cle aved by Rnt1p, followed by exonuclease digestion to U3-3'I and -II, These s pecies are stabilized against continued degradation by binding of Lhp1p. Di splacement of Lhp1p by binding of the snoRNP proteins allows final maturati on, which involves the exosome complex of 3'--> 25' exonucleases.