Symbiotic nitrogen fixation by a nifA deletion mutant of Rhizobium meliloti: the role of an unusual ntrC allele.
In the N2-fixing alfalfa symbiont Rhizobium meliloti, the three sigma 54 (NTRA)-dependent positively acting regulatory proteins NIFA, NTRC, and DCTD are required for activation of promoters involved in N2 fixation (pnifHDKE and pfixABCX), nitrogen assimilation (pglnII), and C4-dicarboxylate transport (pdctA), respectively. Here, we describe an allele of ntrC which results in the constitutive activation of the above NTRC-, NIFA-, and DCTD-regulated promoters. The expression and activation of wild-type NTRC occur in response to nitrogen availability, whereas in cells carrying the ntrC283 allele, the NTRC283 protein appears constitutively active and is constitutively expressed. The ntrC283 allele was shown to carry a single mutation resulting in the replacement of an Asp by a Tyr residue in the helix-turn-helix motif of ntrC283. Introduction of the ntrC283 allele into a nifA deletion mutant restores the N2-fixation ability to 70 to 80% of the wild-type level. Thus, the nifA gene is dispensable for symbiotic N2 fixation.
Labes
M
M
McMaster University, Department of Biology, Hamilton, Ontario, Canada.
Rastogi
V
V
Watson
R
R
Finan
T M
TM
D000483:Alleles; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D002352:Carrier Proteins / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000235:genetics*; D012321:DNA-Directed RNA Polymerases; D029382:Dicarboxylic Acid Transporters; D005786:Gene Expression Regulation; D006651:Histocytochemistry; D000455:Medicago sativa / Q000382:microbiology; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D009586:Nitrogen Fixation / Q000235:genetics*; D051922:PII Nitrogen Regulatory Proteins; D011401:Promoter Regions, Genetic / Q000235:genetics; D017433:Protein Structure, Secondary; D051877:RNA Polymerase Sigma 54; D011994:Recombinant Proteins / Q000096:biosynthesis; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid; D012808:Sigma Factor / Q000378:metabolism; D016962:Sinorhizobium meliloti / Q000235:genetics*; D013489:Suppression, Genetic; D013559:Symbiosis; D015534:Trans-Activators; D014157:Transcription Factors / Q000235:genetics*; D014158:Transcription, Genetic; D001616:beta-Galactosidase / Q000235:genetics / Q000302:isolation & purification
D001426:Bacterial Proteins; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D029382:Dicarboxylic Acid Transporters; C061338:NifA protein, Bacteria; D051922:PII Nitrogen Regulatory Proteins; D011994:Recombinant Proteins; D012808:Sigma Factor; D015534:Trans-Activators; D014157:Transcription Factors; C073786:dctA protein, Sinorhizobium meliloti; C073788:dctD protein, Rhizobium meliloti; D012321:DNA-Directed RNA Polymerases; D051877:RNA Polymerase Sigma 54; D001616:beta-Galactosidase