Nitrogen fixation inpara-nodules of wheat roots by introduced free-living diazotrophs

1991 ◽  
Vol 137 (1) ◽  
pp. 43-47 ◽  
Author(s):  
Y. T. Tchan ◽  
A. M. M. Zeman ◽  
I. R. Kennedy
Keyword(s):  
Nitrogen Fixation ◽  
Free Living ◽  
Wheat Roots

Expression of the nodulation and nitrogen fixation genes in Rhizobium meliloti during development

Genome ◽  
1989 ◽  
Vol 31 (1) ◽  
pp. 354-360 ◽  
Author(s):  
San Chiun Shen ◽  
Shui Ping Wang ◽  
Guan Qiao Yu ◽  
Jia Bi Zhu
Keyword(s):  
Nitrogen Fixation ◽  
Rhizobium Meliloti ◽  
Abnormal Development ◽  
Wild Type ◽  
Free Living ◽  
Nod Genes ◽  
Nodule Initiation ◽  
Fix Genes ◽  
Nitrogen Fixation Genes

Genes that specify nodulation (nod genes) are only active in the free-living rhizobia or in the nodule initiation state of rhizobia. As soon as the repression of nod genes occurs in the bacteroids of the nodule, nifA is induced, while ntrC is inactivated and thus the nifA-mediated nif/fix genes are turned on. Limitation of available oxygen brings about the induction of nifA, which reflects the actual status of nif/fix gene activities in symbiotic state of rhizobia. Oxygen thus appears to be a major symbiotic signal to the expression of bacteroid nif/fix genes. Mutation of nifA or shortage of nifA product in wild-type rhizobia caused by the inhibition of multicopy nifH/fixA promoters leads to an abnormal development of nodules and premature degradation of bacteroids in nodules.Key words: nitrogen fixation, nodulation, nif/fix regulation, nifA mutant.

scholarly journals Differential Regulation of fixN-Reiterated Genes in Rhizobium etli by a Novel fixL—fixK Cascade

2000 ◽  
Vol 13 (12) ◽  
pp. 1283-1292 ◽  
Author(s):  
Lourdes Girard ◽  
Susana Brom ◽  
Araceli Dávalos ◽  
Oswaldo López ◽  
Mario Soberón ◽  
...  
Keyword(s):  
Nitrogen Fixation ◽  
Regulatory Element ◽  
Regulatory Gene ◽  
Regulatory System ◽  
Differential Regulation ◽  
Rhizobium Etli ◽  
Free Living ◽  
Symbiotic Plasmid ◽  
Independent Mechanism ◽  
Fusion Analysis

Among the complexities in the regulation of nitrogen fixation in the Rhizobiaceae are reiteration of regulatory components as well as variant roles for each component between species. For Rhizobium etli CFN42, we reported that the symbiotic plasmid (pCFN42d) contains a key regulatory gene (fixKd) and genes for a symbiotic cytochrome oxidase (fixNOQPd). Here we discuss the occurrence of reiteration of these genes (fixKf and fixNOQPf) and the finding of an unusual fixL homolog on a plasmid previously considered cryptic (pCFN42f). The structure of the deduced FixL polypeptide is suggestive of a fusion of the receiver and transmitter modules of a two-component regulatory system as described in R. leguminosarum bv. viciae VF39. Gene fusion analysis, coupled with mutation of each regulatory element, revealed that free-living expression of FixKf was dependent fully on FixL. In contrast, synthesis of FixKd was not detected under the conditions tested. The FixKf protein is needed for microaerobic expression of both fixN reiterations, whereas the FixKd protein appears to be dispensable. Interestingly, expression of the fixN reiterations exhibits a differential dependence for FixL, where transcription of fixNf was suppressed in the absence of FixL but expression of fixNd still showed significant levels. This suggests the existence of a FixL-independent mechanism for expression of the fixNd reiteration. Surprisingly, mutations in fixL, fixKd, or fixKf (either singly or in combination) did not alter symbiotic effectiveness. A mutation in fixNd (but not in fixNf) was, however, severely affected, indicating a differential role for these reiterations in nitrogen fixation.

scholarly journals Novel European free-living, non-diazotrophic Bradyrhizobium isolates from contrasting soils that lack nodulation and nitrogen fixation genes – a genome comparison

2016 ◽  
Vol 6 (1) ◽  
Author(s):  
Frances Patricia Jones ◽  
Ian M. Clark ◽  
Robert King ◽  
Liz J. Shaw ◽  
Martin J. Woodward ◽  
...  
Keyword(s):  
Nitrogen Fixation ◽  
Genome Comparison ◽  
Free Living ◽  
A Genome ◽  
Nitrogen Fixation Genes

Biological and ecological aspects of nitrogen fixation by free-living micro-organisms

1969 ◽  
Vol 172 (1029) ◽  
pp. 367-388 ◽  
Keyword(s):  
Nitrogen Fixation ◽  
Soil Fertility ◽  
Biological Nitrogen Fixation ◽  
Higher Plants ◽  
Symbiotic Association ◽  
Free Living ◽  
Blue Green Algae ◽  
Ecological Aspects ◽  
Micro Organisms ◽  
Biological Nitrogen

Biological nitrogen fixation is a characteristic of certain micro-organisms, which may be free-living or occur in symbiotic association with higher plants. The purpose of this paper is to summarize some of the biological and ecological aspects of nitrogen-fixation by free-living forms. Biochemical aspects have been reviewed in other contributions to this discussion by Drs Wilson, Burris, and Cox & Fay. Nitrogen fixation by heterotrophic micro-organisms has been considered by Jensen (1965); nitrogen fixation by blue-green algae by Fogg & Stewart (1965), and by Stewart (1966, 1969), while Moore (1966) has evaluated the contribution of nitrogen-fixing micro-organisms to soil fertility.

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