THE CCONOQP GENE-CLUSTER CODES FOR A CB-TYPE CYTOCHROME-OXIDASE THAT FUNCTIONS IN AEROBIC RESPIRATION OF RHODOBACTER-CAPSULATUS

The genes for a new type of a haem-copper cytochrome oxidase were clon ed from Rhodobacter capsulatus strain 37b4, using the Bradyrhizobium j aponicum fixNOQP gene region as a hybridizing probe. Four genes, proba bly organized in an operon (ccoNOQP), were identified; their products share extensive amino acid sequence similarity with the FixN, O, Q and P proteins that have recently been shown to be the subunits oil a cb- type oxidase. CcoN is a b-type cytochrome, CcoO and CcoP are membrane- bound mono- and dihaem c-type cytochromes and CcoQ is a small membrane protein of unknown function. Genes for a similar oxidase are also pre sent in other non-rhizobial bacterial species such as Azotobacter vine landii, Agrobacterium tumefaciens and Pseudomonas aeruginosa, as revea led by polymerase chain reaction analysis, A ccoN mutant was construct ed whose phenotype, in combination with the structural information on the gene products, provides evidence that the CcoNOQP oxidase is a cyt ochrome c oxidase of the cb type, which supports aerobic respiration i n R. capsulatus and which is probably identical to the cbb(3)-type oxi dase that was recently purified from a different strain of the same sp ecies. Mutant analysis also showed that this oxidase has no influence on photosynthetic growth and nitrogen-fixation activity.