HISTIDINE-131, NOT HISTIDINE-43, OF THE BRADYRHIZOBIUM-JAPONICUM FIXNPROTEIN IS EXPOSED TOWARDS THE PERIPLASM AND ESSENTIAL FOR THE FUNCTION OF THE CBB(3)-TYPE CYTOCHROME-OXIDASE

In subunit I (FixN) of the Bradyrhizobium japonicum cbb(3)-type oxidas e, only five instead of the normal six strictly conserved histidines ( H) could be unambiguously assigned as the putative heme or copper liga nds. The ambiguity concerned H43 or H131 as the presumptive N-terminal ligands of the low-spin heme B. We report here that a H43A replacemen t had a wildtype phenotype, whereas the H131A mutant was defective in oxidase function and subunit assembly or stability, suggesting that H1 31 serves as the N-terminal low-spin heme ligand. Topological studies revealed that H131 resides on the periplasmic side of helix 2, where o ne of the low-spin heme ligands is normally found in conventional heme -copper oxidases.