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Is attTn7 insertion site higly conserved?

Goal

To study the level of conservation of the Tn7 transposon recognition site (attTn7) over the phylogeny.

Background

Tn7 is quite a specific bacterial transposon which encodes five transposition proteins (TnS, A, B, C, D and E) (Peters et al, 2001). TnsD is one of the targets selector which inserts Tn7. TnsD recognizes a specific chromosomal site, known as an attachment site (attTn7), and modifies the DNA structure allowing the binding of TnsC, TnsA and TnsB (Waddel and Craig, 1988). These two last proteins insert the transposon. Therefore, insertion takes place preserving expression downstream the essential glmS gene (Gay et al, 1986). It has been reported that the TnsD binding site is found in E.coli in the last 36bp of the glmS ORF (Kuduvalli et al, 2001). This site encodes the active site region of GlmS whose amino acid sequence is nearly completely 100% conserved in all organisms (Milewski, 2002). However, when considering nucleotide sequences, this attTn7 site has not been totally characterized in a wide range of organisms. In 2010, Rupak et al. reported a schematic representation of the nucleotide attTn7 recognition site obtained from the alignment of 25 bacterial glmS genes which have the Tn7 family transposon and the homologous Drosophila, zebrafish and human genes (Rupak et al, 2010).

Figure 1. (from Prasad et al, 2001). Working model for the TnsABC+D pathway. attTn7 is indicated by the rectangle, the relative positions of TnsD, TnsC and the transposase TnsAB are marked by ovals, and the donor DNA is also shown at the insertion site, which is denoted 0. The top portion of the rectangle indicates the major groove side of attTn7 and the bottom side represents the minor groove face of attTn7. Also illustrated in the box is the 5 bp duplication (underlined) that occurs on Tn7 insertion and the arrows show the positions of joining of the transposon ends, executed by the transposase (TnsAB) on attTn7.

Figure 2. The organization of attTn7 (from Rupak et al, 2010). A schematic representation of the attTn7 at the C-terminus of the glmS gene with the TnsD binding site is shown. The sequence of the E. coli GlmS protein, and the consensus attTn7 sequence were derived as described in the text (note that the least conserved nucleotides correspond to the third position of each codon).