Genetic analysis of the cytochrome c‐aa3 branch of the Bradyrhizobium japonicum respiratory chain

M. Bott, M. Bolliger, H. Hennecke

doi:10.1111/j.1365-2958.1990.tb00576.x

<jats:title>Summary</jats:title><jats:p>Further genetic evidence is provided here that <jats:italic>Bradyrhizobium japonieum</jats:italic> possesses a mitochondria‐like electron‐transport pathway: 2[H]→UQ→bc<jats:sub>1</jats:sub>→c→aa<jats:sub>3</jats:sub>→O<jats:sub>2</jats:sub>. Two Tn<jats:italic>5</jats:italic>‐induced mutants, COX122 and COX132, having cytochrome <jats:italic>c</jats:italic> oxidase‐negative phenotypes, were obtained and characterized. Mutant COX122 was defective in a novel gene, named <jats:italic>cycM</jats:italic>, which was responsible for the synthesis of a <jats:italic>c</jats:italic>‐type cytochrome with an <jats:italic>M</jats:italic><jats:sub>r</jats:sub>, of 20000 (20K). This 20K cytochrome <jats:italic>c</jats:italic> appeared to catalyse electron transport from the cytochrome bc<jats:sub>1</jats:sub>, complex to the <jats:italic>aa</jats:italic><jats:sub>3</jats:sub>‐type terminal oxidase and, unlike mitochondrial cytochrome <jats:italic>c</jats:italic>, was membrane‐bound in <jats:italic>B. japonicum.</jats:italic> The Tn<jats:italic>5</jats:italic> insertion of mutant COX132 was localized in <jats:italic>coxA</jats:italic>, the structural gene for subunit I of cytochrome <jats:italic>aa</jats:italic><jats:sub>3</jats:sub>. This finding also led to the cloning and sequencing of the corresponding wild‐type <jats:italic>coxA</jats:italic> gene that encoded a 541 ‐amino‐acid protein with a predicted <jats:italic>M</jats:italic><jats:sub>r</jats:sub> of 59247. The CoxA protein shared about 60% sequence identity with the cytochrome <jats:italic>aa</jats:italic><jats:sub>3</jats:sub> subunit I of mitochondria. The <jats:italic>B. japonicum cycM</jats:italic> and <jats:italic>coxA</jats:italic> mutants were able to fix nitrogen in symbiosis with soybean (Fix<jats:sup>+</jats:sup>). In contrast, mutants described previously which lacked the <jats:italic>bc</jats:italic><jats:sub>1</jats:sub> complex did not develop into endosymbiotic bacteroids and were thus Fix<jats:sup>−</jats:sup>. The data suggest that a symbiosis‐specific respiratory chain exists in <jats:italic>B. japonicum</jats:italic> in which the electrons branch off at the bc<jats:sub>1</jats:sub> complex.</jats:p>

Genetic analysis of the cytochrome <i>c‐aa<sub>3</sub></i> branch of the <i>Bradyrhizobium japonicum</i> respiratory chain

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