AbstractWe have examined some aspects of the biosynthesis of human small nuclear RNAs (snRNAs). The sensitivity of U5 and U4 snRNA synthesis to α‐amanitin in whole cells suggests that RNA polymerase II is involved in the synthesis of these RNA species, in addition to that of U1, U2, and U3 snRNA. Two RNA bands were detected, whose properties are compatible with being U3 and U4 RNA precursors. The cytoplasmic U1 RNA precursor (pU1) was retained by an anti‐RNP antibody column, while the cytoplasmic precursors to U1 and U2 (pU2) RNA were immunoprecipitated by monoclonal anti‐Sm antibodies. Therefore, soon after their transcription, these cytoplasmic RNA precursors assemble with the polypeptides which bear the RNP (pU1) and Sm (pU1 and pU2) antigenic determinants. It has been shown before that, shortly after protein synthesis is interrupted, the apparent cytoplasmic→nuclear transition of newly made U2 RNA is inhibited, while U2 RNA transcription is not. The present data indicate that the trimming of the U2 RNA precursor to mature U2 RNA is not affected early after suppression of protein s
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