Isolation and partial characterization of U1-U6 small RNAs from Bombyx mori

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Abstract
A variety of techniques was used to characterize the U-series small nuclear (Sn) RNA from the posterior silk gland of Bombyx mori. Six molecular species have been identified which correspond to the vertebrate U1-U6 RNA by the following criteria: presence of the RNA in ribonucleoprotein particles which can be immunoprecipitated by lupus Sm antisera; presence of a 2,2,7-trimethylguanosine cap, as assayed by immunoprecipitation with anti-2,2,7-trimethylguanosine IgG; size, as assayed by acrylamide/urea gel electrophoresis using HeLa cell U-RNA markers; and primary nucleotide sequence, as determined by chemical/enzymatic cleavage of end-labeled molecules. The high conservation of primary sequence (66-81% homology based on partial sequences) relative to the corresponding vertebrate U-RNA has permitted unambiguous identification of each molecule. With the exception of 2 subspecies of U3 RNA, the U-snRNA of Bombyx exhibit a striking conservation of secondary structure relative to the proposed structures of the U-RNA of vertebrates. This conservation is best exemplified by several compensatory base alterations that result in the maintenance of hairpin structures. These are particularly evident in U1 and U5 RNA. Bombyx U3 is interesting in that 2 subspecies (of a total of 4 that were sequenced) diverge considerably in sequence (and presumably in structure) relative to the U3 RNA of vertebrates. The most abundant U-RNA in the posterior silk gland appear to be U1 and U2, while U3-U6 are present in relatively small amounts.