NITRATE-INDUCIBLE FORMATE DEHYDROGENASE IN ESCHERICHIA-COLI K-12 .1. NUCLEOTIDE-SEQUENCE OF THE FDNGHI OPERON AND EVIDENCE THAT OPAL (UGA) ENCODES SELENOCYSTEINE

The fdnGHI operon of Escherichia coli encodes nitrate-inducible formate dehydrogenase. We report here the entire nucleotide sequence of fdnGHI. The sequence contains three open reading frames of sizes appropriate to encode the three subunits of formate dehydrogenase-N. fdng contains an in-frame UGA codon that specifies selenocysteine incorporation, and the predicted amino acid sequence of FdnG shows similarity to two other bacterial formate dehydrogenase enzymes. FdnH contains 4 cysteine clusters typical of those found in iron-sulfur proteins. FdnG also contains a cysteine cluster. Evidence from sequence and spectral analyses suggest that FdnI encodes cytochrome b556Fdn. Implications for the membrane topology of formate dehydrogenase-N and its mechanism of proton translocation are discussed.