The eight IS231 variants characterized so far (IS231 A-F, V and W) display similar transposases with an overall 40% identity. Comparison with all the prokaryotic transposable elements sequenced so far revealed that the IS231 transposases share two conserved regions with those of 35 other insertion sequences of wide origins. These insertion sequences, defining the IS4 family, have a common bipartite organization of their ends and are divided into two similarity groups. Interestingly, the transposase domains conserved within this family display similarities with the well known integrase domain shared by transposases of the IS3 and IS15 families, and integrases of retroelements. This domain is also found in IS30-related elements and Tn7 TnsB protein. Amino acid residues conserved throughout all these prokaryotic and eukaryotic mobile genetic elements define a major transposase/integrase motif, likely to play an important role in the transposition process.

The clinically important vancomycin antibiotic inhibits the growth of pathogens such as Staphylococcus aureus by blocking cell wall synthesis through specific recognition of nascent peptidoglycan terminating in D-Ala-D-Ala. Here, we demonstrate the ability of single-molecule atomic force microscopy with antibiotic-modified tips to measure the specific binding forces of vancomycin and to map individual ligands on living bacteria. The single-molecule approach presented here provides new opportunities for understanding the binding mechanisms of antibiotics and for exploring the architecture of bacterial cell walls.

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