HUMAN U19 INTRON-ENCODED SNORNA IS PROCESSED FROM A LONG PRIMARY TRANSCRIPT THAT POSSESSES LITTLE POTENTIAL FOR PROTEIN-CODING

While exons were originally defined as coding regions of split eukaryo tic genes, introns have long been considered as mainly noncoding ''gen etic junk.'' However, recognition that a large number of small nucleol ar RNAs (snoRNAs) are processed from introns of pre-mRNAs demonstrated that introns may also code for functional RNAs. Moreover, recent char acterization of the mammalian UHG gene that encodes eight box C/D intr onic snoRNAs suggested that some genes generate functional RNA product s exclusively from their intron regions. In this study, we show that t he human U19 box H/ACA snoRNA, which is encoded within the second intr on of the U19H gene, represents the only functional RNA product genera ted from the long U19H primary transcript. Splicing of the U19H transc ript, instead of giving rise to a defined RNA, produces a population o f diverse U19H RNA molecules, Although the first three exons of the U1 9H gene are preserved in each processed U19H RNA, the 3' half of the R NA is generated by a series of apparently random splicing events. Beca use the U19H RNA possesses limited potential for protein coding and sh ows a predominant nucleoplasmic localization, we suggest that the sole function of the U19H gene is to express the U19 intronic snoRNA. This suggests that, in marked contrast to our previous dogmatic view, gene s generating functionally important RNAs exclusively from their intron regions are probably more frequent than has been anticipated.