Genes for a microaerobically induced oxidase complex in Bradyrhizobium japonicum are essential for a nitrogen-fixing endosymbiosis.

Abstract

We report the discovery of a Bradyrhizobium

Histidine 131, not histidine 43, of the Bradyrhizobium japonicum FixN protein is exposed towards the periplasm and essential for the function of the cbb3‐type cytochrome oxidase

“…In Bradyrhizobium japonicum the cbb3-type oxidase functions as a cytochrome c oxidase and reduces oxygen to water under microaerobic and symbiotic conditions [1,2]. It belongs to the so-called super-family of heme-copper oxidases [3] and is encoded by the fixNOQP operon [1].…”

“…It belongs to the so-called super-family of heme-copper oxidases [3] and is encoded by the fixNOQP operon [1]. The purified oxidase contains at least three subunits [4].…”

“…The purified oxidase contains at least three subunits [4]. The special structural feature of the cbb3-type oxidase is the association of a subunit I (FixN protein) with two c-type cytochromes, FixO and FixP, representing subunit II and subunit III, respectively, which are anchored in the cytoplasmic membrane by their hydrophobic N-termini and which are exposed to the periplasm [1,2,4,5]. FixN is an integral membrane protein with up to 14 putative transmembrane a-helices [1,2].…”

“…The special structural feature of the cbb3-type oxidase is the association of a subunit I (FixN protein) with two c-type cytochromes, FixO and FixP, representing subunit II and subunit III, respectively, which are anchored in the cytoplasmic membrane by their hydrophobic N-termini and which are exposed to the periplasm [1,2,4,5]. FixN is an integral membrane protein with up to 14 putative transmembrane a-helices [1,2]. It contains a low-and a highspin heine B cofactor, the latter being associated with copper (CUB) in a binuclear center that is the catalytic site of oxygen reduction [3,4,6,7].…”

“…Multiple sequence alignments revealed that the overall amino acid sequence similarity between subunit I of the cbb3-type oxidase and subunit I of the other heme-copper terminal oxidases is low, albeit significant (approx. 20% [1]). In the B. japonicum FixN protein, only five of the six canonical histidines could be readily identified as being conserved (Fig.…”