The two embryonic U1 small nuclear RNAs of Xenopus laevis are encoded by a major family of tandemly repeated genes.

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Abstract
We have identified a large family of U1 RNA genes in Xenopus laevis that encodes two distinct species of U1 RNA. These genes are expressed primarily at the onset of transcription in the 4,000-cell embryo (D. J. Forbes, M. W. Kirschner, D. Caput, J. E. Dahlberg, and E. Lund, Cell 38:681-689, 1984). The two types of embryonic U1 RNA genes are interspersed and are organized in large tandem arrays. The basic 1.9-kilobase repeating unit contains a single copy of each of the embryonic genes and is reiterated ca. 500-fold per haploid genome. This repetitive U1 DNA accounts for more than 90% of all U1 DNA in X. laevis. In addition to this major family, there exist several minor families of dispersed U1 RNA genes, which presumably encode the oocyte and somatic species of X. laevis U1 RNA. Although the embryonic genes are normally inactive in stage VI oocytes, they are expressed when cloned copies are injected into oocyte nuclei.