Bombyx mori 28S ribosomal genes contain insertion elements similar to the Type I and II elements of Drosophila melanogaster.

Abstract

We have examined the 28S ribosomal genes of the silkmoth, Bombyx mori, for the presence of insertion sequences. Two types of insertion sequences were found, each approximately 5 kb in length, which do not share sequence homology. Comparison of the nucleotide sequences of the junction regions with the uninserted gene reveals that one type of insertion has resulted in a 14 bp duplication of the 28S coding region at the insertion site. The location of this insertion and the 14 bp duplication are identical to that found in the Type I ribosomal insertion element of Drosophila melanogaster. The second type of insertion element is located at a site corresponding to approximately 75 bp upstream of the first type. The location of this insertion, the variability detected at its 5′ junction, and a short region of sequence homology at its 3′ junction suggest that it is related to the Type II element of D. melanogaster. This is the first example of a Type II‐like rDNA insertion outside of sibling species of D. melanogaster, and the first example of a Type I‐like rDNA insertion outside of the higher Diptera.

The site-specific ribosomal insertion element type II of Bombyx mori (R2Bm) contains the coding sequence for a reverse transcriptase-like enzyme.

“…From our previously characterized collection of lambda clones containing uninterrupted or interrupted B. mori rDNA units (12,25), clone B131 was selected for further analysis because it contained a complete R2 element. A 6.0-kb BamHI-XbaI fragment of B131 containing the 4.2-kb R2 element, 0.3 kb of flanking 28S gene sequences at the 5' junction of the element, and 1.5 kb of 28S gene sequences at the 3' junction of the element was subcloned, and a detailed restriction map was generated (Fig.…”

“…In the case of the 3' junction of the R2 element, four copies of the element have been sequenced ( 12]). All the copies end with the sequence GAAAA (Fig.…”

“…Lambda clones containing rDNA units with R2 elements (B74, B98, and B131) or without this element (B108) were isolated from a Charon 4 library made from genomic DNA partially digested with EcoRI (11). The description of these clones and the sequence of their rDNA-R2 junction regions have been previously described (12). Clones B701, B702, and B703 were derived from a lambda library of partial Sau3A-I-digested genomic DNA cloned into Charon 35 (a gift from B. Hibner).…”

“…They interrupt the 28S rRNA gene at a location approximately two-thirds of the distance from the 5' end of the gene and are flanked by a 14-base-pair (bp) duplication of rDNA sequences. Type II elements have only been reported in D. melanogaster (7,37) and B. mori (12,16). They interrupt the 28S gene 75 bp upstream of the type I elements and do not contain flanking duplications of rDNA sequences.…”

“…Based on the nucleotide sequences of their junction regions, these insertions have been divided into two classes. Type I elements have been reported in Drosophila virilis (34), D. melanogaster (7,37), Calliphora erythrocephala (43), and Bombyx mori (12,16). They interrupt the 28S rRNA gene at a location approximately two-thirds of the distance from the 5' end of the gene and are flanked by a 14-base-pair (bp) duplication of rDNA sequences.…”