SULFIDE-QUINONE REDUCTASE FROM RHODOBACTER-CAPSULATUS - PURIFICATION,CLONING, AND EXPRESSION

A sulfide-quinone oxidoreductase (SQR, EC 1.8.5.'.) has been purified to homogeneity from chromatophores of the non-sulfur purple bacterium Rhodobacter capsulatus DSM 155. It is composed of a single polypeptide with an apparent molecular mass of about 55 kDa, exhibiting absorptio n and fluorescence spectra typical for a flavoprotein and similar to t he SQR from the cyanobacterium Oscillatoria limnetica. From N-terminal and tryptic peptide sequences of the pure protein a genomic DNA clone was obtained by polymerase chain reaction amplification. Its sequence contains an open reading frame of 1275 base pairs (EMBL nucleotide se quence data base, accession no, X97478) encoding the SQR of R. capsula tus. The deduced polypeptide consists of 425 amino acid residues with a molecular mass of 47 kDa and a net charge of +9. The high similarity (72%)/identity (48%) between the N termini of the cyanobacterial and the bacterial enzyme was confirmed and extended. Both enzymes exhibit the FAD/NAD(P) binding beta alpha beta-fold (Wierenga, R. K., Terpstra , P., and Hol, W. G. S. (1986) J. Mol. Biol. 187, 101-107). The comple te sequence of the SQR from R. capsulatus shows further similarity to flavoproteins, in particular glutathione reductase and lipoamide dehyd rogenase. The cloned sqr was expressed in Escherichia coli in a functi onal form.