Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710.
The cDNA encoding sulfite oxidase has been cloned from a rat liver cDNA library. The gene contains a single open reading frame of 1464 nucleotides encoding a protein of 488 amino acids. The deduced amino acid sequence contains a 22-residue amino-terminal presequence that may serve as a mitochondrial targeting signal. The amino acid sequence deduced from the cDNA shows significant similarity to those of sulfite oxidase from chicken liver and nitrate reductases from algal, fungal, and plant sources. Two cysteine residues are conserved in all of these proteins, and it is proposed that one or both of these cysteines serve as ligands to molybdenum. The gene has been expressed in Escherichia coli to a level equivalent to that observed in rat liver. The recombinant enzyme has been found to contain the molybdopterin form of the molybdenum cofactor and is active as determined by the sulfite dependent reduction of cytochrome c.