Defective gamma subunit of ATP synthase (F1F0) from Escherichia coli leads to resistance to aminoglycoside antibiotics

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Abstract
A strain of Escherichia coli which was derived from a gentamicin-resistant clinical isolate was found to be cross-resistant to neomycin and streptomycin. The molecular nature of the genetic defect was found to be an insertion of two GC base pairs in the uncG gene of the mutant. The insertion led to the production of a truncated gamma subunit of 247 amino acids in length instead of the 286 amino acids that are present in the normal gamma subunit. A plasmid which carried the ATP synthase genes from the mutant produced resistance to aminoglycoside antibiotics when it was introduced into a strain with a chromosomal deletion of the ATP synthase genes. Removal of the genes coding for the beta and epsilon subunits abolished antibiotic resistance coded by the mutant plasmid. The relationship between antibiotic resistance and the gamma subunit was investigated by testing the antibiotic resistance of plasmids carrying various combinations of unc genes. The presence of genes for the F0 portion of the ATP synthase in the presence or absence of genes for the gamma subunit was not sufficient to cause antibiotic resistance. alpha, beta, and truncated gamma subunits were detected on washed membranes of the mutant by immunoblotting. The first 247 amino acid residues of the gamma subunit may be sufficient to allow its association with other F1 subunits in such a way that the proton gate of F0 is held open by the mutant F1.