No abstract

The DNA sequence and derived amino-acid sequence of a 5618-base region in the 74-min area of the Escherichia coli chromosome has been determined in order to locate the structural gene, nirB, for the NADH-dependent nitrite reductase and a gene, cysG, required for the synthesis of the sirohaem prosthetic group. Three additional open reading frames, nirD, nirE and nirC, were found between nirB and cysG.Potential binding sites on the NirB protein for NADH and FAD, as well as conserved central core and interface domains, were deduced by comparing the derived amino-acid sequence with those of database proteins. A directly repeated sequence, which includes the motif -Cys-Xaa-Xaa-Cys-, is suggested as the binding site for either oneThe nirD gene potentially encodes a soluble, cytoplasmic protein of unknown function. No significant similarities were found between the derived amino-acid sequence of NirD and either NirB or any other protein in the database. If the nirE open reading frame is translated, it would encode a 33-amino-acid peptide of unknown function which includes 8 phenylalanyl residues.The product of the nirC gene is a highly hydrophobic protein with regions of amino-acid sequence similar to cytochrome oxidase polypeptide 1.Two genes essential for the major NADH-dependent nitrite reductase activity are located in the 74-min region of the Escherichiu coli K-12 chromosome [l]. They are nirB, the structural gene for the nitrite reductase apoprotein, and cysG, which is required for sirohaem synthesis. The prosthetic groups of nitrite reductase are FAD, an iron-sulphur cluster and sirohaem which is also found in the NADPH-dependent sulphite reductase. Mutants defective in the cysG gene are unable to grow without cysteine, or to reduce nitrite rapidly to ammonia [2,. The cysG product has recently been purified and characterized (C. Roessner, personal communication). It catalyses two methylation reactions in the conversion of uroporphyrinogen I11 into sirohaem.We have previously located the promoter, transcription and translation start points of the nirB gene and reported the DNA sequence and derived amino-acid sequence of the first 89 bases of nirB and its 5'-regulatory region [4]. We now report Correspondence to J. A. Cole,