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We showed previously that those U1 small nuclear RNA (snRNA) genes of Xenopus laevis which are transcribed very actively in early embryos are quiescent in mature (stage VI) oocytes (Forbes et al. 1984). Although that study demonstrated that differential control of snRNA genes occurred, it did not describe snRNA accumulation during development. Using high-resolution polyacrylamide gels in combination with Northern blot hybridization and RNA sequence analyses, we show here that Xenopus has at least three classes of U1 and U4 snRNAs that are distinguishable by their differential expression of oocytes, embryos, tadpoles, and frogs. Adult snRNAs appear to be synthesized constitutively throughout Xenopus development and comprise the major species in tissues from large tadpoles and frogs. Embryonic snRNAs are the principal species accumulating during the two periods of rapid snRNA synthesis, i.e., in previtellogenic oocytes and early embryos. Tadpole RNAs are minor species that are most prominent in young feeding tadpoles. Transcription of both embryonic and adult snRNA genes is activated at the midblastula transition (MBT), but expression of the embryonic genes is switched off selectively within a few days after MBT. Although the precise timing of this inactivation differs significantly for U1 and U4 genes, the overall pattern of differential expression is common to U1 and U2 snRNA genes. Because of sequence differences between the snRNAs accumulating at various stages, the resulting populations of snRNPs could have different splice-site specificities leading to altered patterns of pre-mRNA splicing during development.

Differential accumulation of U1 and U4 small nuclear RNAs during Xenopus development.