Thiophene-degrading Escherichia coli mutants possess sulfone oxidase activity and show altered resistance to sulfur-containing antibiotics

We have previously isolated mutants of Escherichia coli which show increased oxidation of heterocyclic furan and thiophene substrates. We have now found that strains carrying the thdA mutation express a novel enzyme activity which oxidizes a variety of substrates containing a sulfone (SO2) moiety. Both heterocyclic sulfones (e.g., tetramethylene sulfone) and simple aliphatic sulfones (e.g., ethyl sulfone) were oxidized. The thdA mutants were more resistant than wild-type strains to aromatic sulfone antibiotics such as dapsone. In contrast they showed increased susceptibility to thiolutin, a cyclic antibiotic containing sulfur at the sulfide level of oxidation. Several new thdA mutant alleles were isolated by selecting for increased oxidation of various aliphatic sulfur compounds. These new thdA mutants showed similar sulfone oxidase activity and the same map location (at 10.7 min) as the original thdA1 mutation. The constitutive fadR mutation was required for the phenotypic expression of thdA-mediated oxidation of sulfur compounds. However, the thdA-directed expression of sulfone oxidase activity was not fadR dependent. The thdC and thdD mutations probably protect against the toxicity of thiophene derivatives rather than conferring improved metabolic capability.