A Novel Tripartite Structure Comprising a Mariner-Like Element and Two Additional Retrotransposons Found in the Bombyx mori Genome

Abstract

A clone of a DNA-mediated mobile element (transposon) corresponding to a mariner-like element (MLE) was obtained by carrying out the polymerase chain reaction with genomic DNA of Bombyx mori using a Hyalophora cecropia MLE sequence as a primer. This clone had a size of about 4.2 kb and, after sequencing, was found to contain an RNA-mediated, shorter retrotransposon named L1Bm, 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 conserved sequence which was subsequently found to be highly conserved in all BMC1 and L1Bm elements registered so far. We propose that the 63-bp stretch may be a recognition site for a retrotransposition mechanism conducted by a reverse transcriptase and an endonuclease complex. On the basis of this inference, we propose a model that predicts how different types of BMC1 and L1Bm elements are dispersed in the genome. In addition, a phylogenetic tree made from the current and extant BMC1 and L1Bm sequences indicated that these elements can be classified into Subfamilies I and II.

The Old and the Restless

“…To date, the relationship between V(D)J recombination and a mobile element has been discussed largely in terms of familiar retroviruses and of transposons such as Mu and Tn10 56716, but another chapter to the story will unfold when/if we can discover more about the exact sort of mobile element the “RAG transposon” actually was. Perhaps it was designed to mobilize or rapidly generate flocks of genes rather than singular units, or perhaps at some point in its evolution, the RAG transposon was a conglomerate generated in a pile-up of more than one type of mobile element (e.g., reference 17). Meanwhile, sharks, as animals in which such RAG-mediated experimentation is still apparently ongoing, are likely to prove a particularly fruitful system in which to further explore such questions.…”

Profile of the mosaic element BTMR1 in the genome of the bumble bee Bombus terrestris (Hymenoptera: Apidae)

“…Besides their domestication or maintenance as active, inactive, or domesticated copies, the ‘sympatry’ of TEs in the nuclear environment enables new mosaic elements that are active in transposition to emerge. To date, such composite elements have been reported in several taxa, and result from recombination and transposition events between Class‐I retrotransposons (Li et al ., 2009), a retrotransposon and a gene, non‐TE repeats such as satellite DNA or variable number of tandem repeats (VNTR; Fudal et al ., 2005; Wang et al ., 2005; Souza et al ., 2007), or between Class‐I and Class‐II elements (Nakajima et al ., 1999; Palomeque et al ., 2006). Depending on how they originated, these composite elements can move by either retrotransposition or transposition, the hallmark of their mobility being in general that they are flanked by a target site duplication (TSD).…”