A novel tripartite structure comprising a mariner-like element and two additional retrotransposons found in the Bombyx mori genome

A clone of a DNA-mediated mobile element (transposon) corresponding to a ma riner-like element (MLE) was obtained by carrying out the polymerase chain reaction with genomic DNA of Bombyx mori using a Hyalophora cecropia MLE se quence as a primer, This clone had a size of about 4.2 kb and, after sequen cing, was found to contain an RNA-mediated, shorter retrotransposon named L 1Bm, which was in turn integrated with a much longer retrotransposon named BMC1, Thus, the mobile elements made a novel tripartite structure. The BMC1 and L1Bm moieties of the composite structure each contained a 63-bp conser ved sequence which was subsequently found to be highly conserved in all BMC 1 and L1Bm elements registered so far. We propose that the 63-bp stretch ma y be a recognition site for a retrotransposition mechanism conducted by a r everse transcriptase and an endonuclease complex. On the basis of this infe rence, we propose a model that predicts how different types of BMC1 and L1B m elements are dispersed in the genome. In addition, a phylogenetic tree ma de from the current and extant BMC1 and L1Bm sequences indicated that these elements can be classified into Subfamilies I and II.