MULTIPLE PATHWAYS OF ELECTRON-TRANSFER IN DIMETHYL-SULFOXIDE REDUCTASE OF ESCHERICHIA-COLI

The catalytic subunit of dimethyl sulfoxide (Me(2)SO) reductase, DmsA, contains six blocks of sequence that are homologous to other members of the superfamily of prokaryotic molybdoenzymes. The amino-terminal b lock contains 5 conserved residues (Cys(38), Cys(42), Cys(75) Lys(28), and Arg(77)). Site-directed mutagenesis of these residues did not alt er membrane localization but in some cases less enzyme accumulated. Th e activity of Me(2)SO reductase was monitored by measuring Me(2)SO-dep endent anaerobic growth, benzyl viologen, or dimethylnaphthoquinol oxi dase activity, and using a quinol pool-coupling assay. Only Cys(75) an d Lys(28) mutant enzymes were able to support anaerobic growth with Me (2)SO suggesting a critical role for Cys(38), Cys(42), and Arg(77). Be nzyl viologen oxidase activity was retained in the mutants although wi th reduced efficiency in Cys(42)-Ser. Electron transport with dimethyl naphthoquinol was reduced in Cys(38)-Ser, Cys(42)-Ser, and Cys(75)-Ser and almost totally eliminated in the Arg(77)-Ser mutant. Cys(38)-Ser, Cys(42)-Ser, and Arg(77)-Ser were unable to support quinol oxidation although electron transfer from the quinol pool to the [Fe-S] centers in DmsB was normal. These results indicate that the amino-terminal reg ion is involved in functional electron transfer from the quinol pool t o Me(2)SO and that electrons from benzyl viologen, dimethylnaphthoquin ol, and menaquinol may follow different paths within the catalytic sub unit.