A 2ND TERMINAL OXIDASE IN SULFOLOBUS-ACIDOCALDARIUS

We previously found that the soxABCD operon encodes a quinol oxidase c omplex in Sulfolobus acidocaldarius and this enzyme was purified and c haracterized. In this study, we have used a cloning procedure based on the conservation of oxidase sequences and the polymerase chain reacti on to isolate a new gene (soxM) encoding a subunit of another terminal oxidase. This terminal oxidase is a fusion between two central compon ents of cytochrome oxidases, subunits I and III. soxM forms a transcri ptional unit which is expressed under heterotrophic growth conditions. The corresponding protein was detected by direct protein sequencing i n a preparation enriched with a cytochrome absorbing light at 562 nm. This preparation contains a terminal oxidase which is able to oxidize the artificial substrate N,N,N',N'-tetramethyl-p-phenylenediamine This preparation also contains SoxC, a protein homologous to the mitochond rial cytochrome b, and a Rieske iron-sulphur center. We suggest that S oxM is the core component of a second terminal oxidase complex and tha t this complex may share a subunit (SoxC) with the SoxABCD complex.