Evidence for a relationship between the Bombyx mori middle repetitive Bm1 sequence family and U1 snRNA

Several genomic library equivalents of Bombyx mori were constructed in the EMBL-4 lambda derivative. The genomic bank was screened with purified Bombyx mori U1 RNA and twenty positive clones for the U1 gene were isolated. Three U1-related sequences were subcloned and sequenced. Two of the sequences are U1 pseudogenes while a third sequence represents a member of the Bm 1 family of repetitive elements of B.mori with significant sequence similarity to U1 small nuclear RNA. The U1-related Bm1 element exhibits 82% sequence similarity with the Bm1 consensus sequence and, under less stringent computer comparison parameters, 60% similarity with a composite B.mori/Drosophila melanogaster U1 gene. The Bm1 family consensus sequence exhibits 53% sequence similarity with the composite U1 gene. The two pseudogenes possess highly conserved sequences with the B.mori U1 gene only for the first 101 nucleotides. These findings are indicative of at least two different categories of U1-related sequences in B.mori, one with a possible evolutionary relationship to the Bm1 family of repetitive elements and the other representing characteristic processed pseudogenes with retroposon mode of dispersion and target selection for the TTTA hotspot. In addition, the U1-related Bm1 element may demonstrate for the first time that a family of retroposons is ultimately derived from a U snRNA.