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Genetic analysis of the cytochrome c‐aa 3 branch of the Bradyrhizobium japonicum respiratory chain
Author(s) -
Bott M.,
Bolliger M.,
Hennecke H.
Publication year - 1990
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1111/j.1365-2958.1990.tb00576.x
Subject(s) - biology , cytochrome c oxidase , mutant , cytochrome b , bradyrhizobium japonicum , cytochrome , cytochrome c , protein subunit , coenzyme q – cytochrome c reductase , respiratory chain , cytochrome c1 , microbiology and biotechnology , biochemistry , gene , mitochondrion , genetics , mitochondrial dna , rhizobiaceae , enzyme , symbiosis , bacteria
Summary Further genetic evidence is provided here that Bradyrhizobium japonieum possesses a mitochondria‐like electron‐transport pathway: 2[H]→UQ→bc 1 →c→aa 3 →O 2 . Two Tn 5 ‐induced mutants, COX122 and COX132, having cytochrome c oxidase‐negative phenotypes, were obtained and characterized. Mutant COX122 was defective in a novel gene, named cycM , which was responsible for the synthesis of a c ‐type cytochrome with an M r , of 20000 (20K). This 20K cytochrome c appeared to catalyse electron transport from the cytochrome bc 1 , complex to the aa 3 ‐type terminal oxidase and, unlike mitochondrial cytochrome c , was membrane‐bound in B. japonicum. The Tn 5 insertion of mutant COX132 was localized in coxA , the structural gene for subunit I of cytochrome aa 3 . This finding also led to the cloning and sequencing of the corresponding wild‐type coxA gene that encoded a 541 ‐amino‐acid protein with a predicted M r of 59247. The CoxA protein shared about 60% sequence identity with the cytochrome aa 3 subunit I of mitochondria. The B. japonicum cycM and coxA mutants were able to fix nitrogen in symbiosis with soybean (Fix + ). In contrast, mutants described previously which lacked the bc 1 complex did not develop into endosymbiotic bacteroids and were thus Fix − . The data suggest that a symbiosis‐specific respiratory chain exists in B. japonicum in which the electrons branch off at the bc 1 complex.