Intronic U14 snoRNAs of Xenopus laevis are located in two different parent genes and can be processed from their introns during early oogenesis

U14 is a member of the rapidly growing family of intronic small nucleolar RNAs (snoRNAs) that are involved in pre-rRNA processing and ribosome biogenesis, These snoRNA species are encoded within introns of eukaryotic protein coding genes and are synthesized via an intron processing pathway. Characterization of Xenopus laevis U14 snoRNA genes has revealed that in addition to the anticipated location of U14 within introns of the amphibian hsc70 gene (introns 4, 5 and 7), additional intronic U14 snoRNAs are also found in the ribosomal protein S13 gene (introns 3 and 4). U14 is thus far a unique intronic snoRNA in that it is encoded within two different parent genes of a single organism. Northern blot analysis revealed that U14 snoRNAs accumulate during early oocyte development and are rapidly expressed after the mid-blastula transition of developing embryos. Microinjection of hsc70 pre-mRNAs into developing oocytes demonstrated that oocytes as early as stages II and III are capable of processing U14 snoRNA from the pre-mRNA precursor, The ability of immature oocytes to process intronic snoRNAs is consistent with the observed accumulation of U14 during oocyte maturation and the developmentally regulated synthesis of rRNA during oogenesis.