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Novel Reiterated Fnr-Type Proteins Control the Production of the Symbiotic Terminal Oxidase cbb₃ in Rhizobium etli CFN42

¹Programa de Genómica Funcional de Procariotes and ²Programa de Ingeniería Genómica, Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Ap. Postal 565-A, Cuernavaca, Morelos, 62271, México; ³Departamento de Microbiología del Suelo y Sistemas Simbióticos. Estación Experimental del Zaidín, CSIC. Ap. Postal 419, 18080 Granada, España

Symbiotic nitrogen-fixing bacteria express a terminal oxidase with a high oxygen affinity, the cbb₃-type oxidase encoded by the fixNOQP operon. Previously, we have shown that, in Rhizobium etli CFN42, the repeated fixNOQP operons (fixNOQPd and fixNOQPf) have a differential role in nitrogen fixation. Only the fixNOQPd operon is required for the establishment of an effective symbiosis; microaerobic induction of this operon is under the control of at least three transcriptional regulators, FixKf, FnrNd, and FnrNchr, belonging to the Crp/Fnr family. In this work, we describe two novel Crp/Fnr-type transcriptional regulators (StoRd and StoRf, symbiotic terminal oxidase regulators) that play differential roles in the control of key genes for nitrogen fixation. Mutations either in stoRd or stoRf enhance the microaerobic expression of both fixNOQP reiterations, increasing also the synthesis of the cbb₃-type oxidase in nodules. Despite their structural similarity, a differential role of these genes was also revealed, since a mutation in stoRd but not in stoRf enhanced both the expression of fixKf and the nitrogen-fixing capacity of R. etli CFN42.