SUMMARY: Two mutant strains of K12 Doc-S resistant to the uncoupling agents 4,5,6,7- tetrachloro-2-trifluoromethyl benzimidazole and carbonyl cyanide -cblorophenylhydrazone were isolated. These strains, designated TUV and CUV, were capable of () growth, () the transport of succinate and -proline and () electron-transport-linked oxidative synthesis of ATP in the presence of titres of uncoupler which inhibited these processes in strain Doc-S. The inhibition of transport of L-proline by a fixed titre of uncoupler was sharply pH dependent in strain Doc-S: uptake was unaffected at pH 7.6 but completely inhibited at pH 5.6. This pH dependence was not shown by the resistant strains. We believe that uncouplers were equally accessible to their site(s) of action in the energy-conserving membrane of the sensitive and resistant strains. We conclude that uncoupler resistance in these strains of has arisen as a consequence of mutations which directly affect a specific site of uncoupler action within the cytoplasmic membrane, rather than as a consequence of a decrease in the permeability of cells to uncoupler.


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