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The Xenopus egg and embryo, throughout the transcriptionally inactive early cleavage period, were found to contain a store of approximately 8 X 10(8) molecules of the small nuclear RNA (snRNA) U1, sufficient for 4,000-8,000 nuclei. In addition, when transcription is activated at the twelfth cleavage (4,000 cell-stage), the snRNAs U1, U2, U4, U5, and U6 are major RNA polymerase II products. From the twelfth cleavage to gastrulation, U1 RNA increases sevenfold in 4 h, paralleling a similar increase in nuclear number. This level of snRNA transcription is much greater than that typical of somatic cells, implying a higher rate of U1 transcription or a greater number of U1 genes active in the embryo. The Xenopus egg also contains snRNP proteins, since it has the capacity to package exogenously added snRNA into immunoprecipitable snRNP particles, which resemble endogenous particles in both sedimentation coefficient and T1 RNase digestibility. SnRNP proteins may recognize conserved secondary structure of U1 snRNA since efficient packaging of both mouse and Drosophila U1 RNAs, differing 30% in sequence, occurs. The Xenopus egg and embryo can be used to pose a number of interesting questions about the transcription, assembly, and function of snRNA.
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