Regulation of gene expression in response to oxygen in Rhizobium etli: role of FnrN in fixNOQP expression and in symbiotic nitrogen fixation.
Previously, we reported finding duplicated fixNOQP operons in Rhizobium etli CFN42. One of these duplicated operons is located in the symbiotic plasmid (fixNOQPd), while the other is located in a cryptic plasmid (fixNOQPf). Although a novel FixL-FixKf regulatory cascade participates in microaerobic expression of both fixNOQP duplicated operons, we found that a mutation in fixL eliminates fixNOQPf expression but has only a moderate effect on expression of fixNOQPd. This suggests that there are differential regulatory controls. Interestingly, only the fixNOQPd operon was essential for symbiotic nitrogen fixation (L. Girard, S. Brom, A. Dávalos, O. Lopez, M. Soberón, and D. Romero, Mol. Plant-Microbe Interact. 13:1283-1292, 2000). Searching for potential candidates responsible for the differential expression, we characterized two fnrN homologs (encoding transcriptional activators of the cyclic AMP receptor protein [CRP]-Fnr family) in R. etli CFN42. One of these genes (fnrNd) is located on the symbiotic plasmid, while the other (fnrNchr) is located on the chromosome. Analysis of the expression of the fnrN genes using transcriptional fusions with lacZ showed that the two fnrN genes are differentially regulated, since only fnrNd is expressed in microaerobic cultures of the wild-type strain while fnrNchr is negatively controlled by FixL. Mutagenesis of the two fnrN genes showed that both genes participate, in conjunction with FixL-FixKf, in the microaerobic induction of the fixNOQPd operon. Participation of these genes is also seen during the symbiotic process, in which mutations in fnrNd and fnrNchr, either singly or in combination, lead to reductions in nitrogen fixation. Therefore, R. etli employs a regulatory circuit for induction of the fixNOQPd operon that involves at least three transcriptional regulators of the CRP-Fnr family. This regulatory circuit may be important for ensuring optimal production of the cbb(3), terminal oxidase during symbiosis.
Lopez
O
O
Departamento de Microbiología Molecular, Instituto de Biotecnología, UNAM, Cuernavaca, Morelos, 62250, Mexico.
Morera
C
C
Miranda-Rios
J
J
Girard
L
L
Romero
D
D
Soberón
M
M
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000096:biosynthesis* / Q000378:metabolism*; D003574:Cytochrome c Group / Q000096:biosynthesis*; D020440:Gene Duplication; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D005809:Genes, Regulator; D006420:Hemeproteins / Q000378:metabolism; D000071677:Histidine Kinase; D008565:Membrane Proteins / Q000096:biosynthesis; D008957:Models, Genetic; D008969:Molecular Sequence Data; D009154:Mutation; D009586:Nitrogen Fixation / Q000235:genetics*; D009591:Nitrogenase / Q000235:genetics / Q000378:metabolism; D009876:Operon; D010100:Oxygen / Q000494:pharmacology*; D027805:Phaseolus / Q000382:microbiology; D012231:Rhizobium / Q000187:drug effects / Q000235:genetics*; D017386:Sequence Homology, Amino Acid; D013559:Symbiosis / Q000235:genetics; D014157:Transcription Factors
D001426:Bacterial Proteins; D003574:Cytochrome c Group; C087009:FixN protein, bacteria; C080744:FixQ protein, Bacteria; C095881:FnrN protein, Rhizobium leguminosarum; D006420:Hemeproteins; D008565:Membrane Proteins; D014157:Transcription Factors; C080743:cytochrome FixO; C080742:cytochrome FixP; C066628:FixK protein, Bacteria; D009591:Nitrogenase; C067929:FixL protein, Bacteria; D000071677:Histidine Kinase; D010100:Oxygen