This work addresses the biogenesis of heme-copper terminal oxidases in Bradyrhizobium japonicum, the nitrogen-fixing root nodule symbiont of soybean. B. japonicum has four quinol oxidases and four cytochrome oxidases. The latter include the aa 3 -and cbb 3 -type oxidases. Although both have a Cu B center in subunit I, the subunit II proteins differ in having either a Cu A center (in aa 3 ) or a covalently bound heme c (in cbb 3 ). Two biogenesis factors were genetically studied here, the periplasmically exposed CoxG and ScoI proteins, which are the respective homologs of the mitochondrial copper-trafficking chaperones Cox11 and Sco1 for the formation of the Cu B center in subunit I and the Cu A center in subunit II of cytochrome aa 3 . We could demonstrate copper binding to ScoI in vitro, a process for which the thiols of cysteine residues 74 and 78 in the ScoI polypeptide were shown to be essential. Knock-out mutations in the B. japonicum coxG and scoI genes led to loss of cytochrome aa 3 assembly and activity in the cytoplasmic membrane, whereas the cbb 3 -type cytochrome oxidase apparently remained unaffected. This suggests that subunit I of the cbb 3 -type oxidase obtains its copper cofactor via a different pathway than cytochrome aa 3 . In contrast to the coxG mutation, the scoI mutation caused a decreased symbiotic nitrogen fixation activity. We hypothesize that a periplasmic B. japonicum protein other than any of the identified Cu A proteins depends on ScoI and is required for an effective symbiosis.
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