Aminoglycoside resistance resulting from tight drug binding to an altered aminoglycoside acetyltransferase

The aacA29b gene, which confers an atypical aminoglycoside resistance pattern to Escherichia coli, was identified on a class 1 integron from a multidrug-resistant isolate of Pseudomonas aeruginosa. On the basis of amino acid sequence homology, it was proposed that the gene encoded a 6′-N-acetyltransferase. The resistance gene was cloned into the pET23a(+) vector, and overexpression conferred high-level resistance to the usual substrates of the aminoglycoside N-acetyltransferase AAC(6′)-I, except netilmicin. The level of resistance conferred by aacA29b correlated perfectly with the level of expression of the gene. The corresponding C-terminal six-His-tagged AAC(6′)-29b protein was purified and found to exist as a dimer in solution. With a spectrophotometric assay, an extremely feeble AAC activity was detected with acetyl coenzyme A (acetyl-CoA) as an acetyl donor. Fluorescence titrations of the protein with aminoglycosides demonstrated the very tight binding of tobramycin, dibekacin, kanamycin A, sisomicin (K_d, ≤1 μM) and a weaker affinity for amikacin (K_d, ≈60 μM). The binding of netilmicin and acetyl-CoA could not be detected by either fluorescence spectroscopy or isothermal titration calorimetry. The inability of AAC(6′)-29b to efficiently bind acetyl-CoA is supported by an alignment analysis of its amino acid sequence compared with those of other AAC(6′)-I family members. AAC(6′)-29b lacks a number of residues involved in acetyl-CoA binding. These results lead to the conclusion that AAC(6′)-29b is able to confer aminoglycoside resistance by sequestering the drug as a result of tight binding.