HEME-C INCORPORATION INTO THE C-TYPE CYTOCHROMES FIXO AND FIXP IS ESSENTIAL FOR ASSEMBLY OF THE BRADYRHIZOBIUM-JAPONICUM CBB(3)-TYPE OXIDASE

The monoheme and diheme c-type cytochromes FixO and FixP are two of th e subunits of the respiratory cbb(3)-type oxidase of Bradyrhizobium ja ponicum. The cysteines of the respective heme C binding motifs CXXCH w ere changed to serines by site-directed mutagenesis, which led to inac tive oxidases in all mutants. Western blot analyses showed that an int act heme binding site in the FixO polypeptide is a prerequisite not on ly for the synthesis of holo-FixO protein but also for the formation o f the entire cbb(3)-type oxidase complex. Both heme binding sites of F ixP were essential for maturation and assembly of this subunit. It was not possible to create stable FixP variants that contained only one h eme C. (C) 1997 Federation of European Biochemical Societies.