Dimitris Georgellis,3 and
1Programa de Genómica Funcional de Procariotes, Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Avenida Universidad 1001, Cuernavaca, Morelos, 62209, México; 2National Center for Soybean Biotechnology, Division of Plant Sciences, University of Missouri, Columbia 65211, U.S.A.; 3Departamento de Genética Molecular, Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, Ciudad Universitaria, México D.F. 04510, México
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Accepted 8 July 2012.
Transcriptional control of the fixK gene in Rhizobium etli and R. leguminosarum bv. viciae is governed by a two-component signal transduction system that diverts from the conventional FixL-FixJ cascade that occurs in model rhizobia. Although a fixL gene, encoding a hybrid histidine kinase (hFixL), is present in R. etli, no fixJ, the cognate response regulator, has been identified. In this work, we present evidence that the pRet42f-located open reading frame RHE_PF00530 (fxkR) encodes a novel response regulator indispensable for fixKf activation under microaerobic growth. Moreover, results from complementation assays demonstrate that the activation of fixKf expression requires the presence of both hFixL and FxkR, and that the fxkR ortholog from R. leguminosarum bv. viciae is able to substitute for FxkR transcriptional control in R. etli. In addition, in these two organisms, hFixL- and FxkR-related proteins were identified in other bacteria, located in close proximity to a fixK-related gene. Using reporter fusions, site-directed mutagenesis, and electrophoretic mobility shift assays, we identified the FxkR binding site upstream from the transcriptional start site of fixKf. Similar to our previous observations for fixL and fixKf mutants, a null mutation in fxkR does not affect the symbiotic effectiveness of the strain. Thus, our findings reveal that FxkR is the long-standing missing key regulator that allows the transduction of the microaerobic signal for the activation of the FixKf regulon.
© 2012 The American Phytopathological Society