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A phosphate transport system is required for symbiotic nitrogen fixation by Rhizobium meliloti
Abstract:
The bacterium Rhizobium meliloti forms N2-fixing root nodules on alfalfa plants. The ndvF locus, located on the 1,700-kb pEXO megaplasmid of R. meliloti, is required for nodule invasion and N2 fixation. Here we report that ndvF contains four genes, phoCDET, which encode an ABC-type transport system for the uptake of Pi into the bacteria. The PhoC and PhoD proteins are homologous to the Escherichia coli phosphonate transport proteins PhnC and PhnD. The PhoT and PhoE proteins are homologous to each other and to the E. coli phosphonate transport protein PhnE. We show that the R. meliloti phoD and phoE genes are induced in response to phosphate starvation and that the phoC promoter contains two elements which are similar in sequence to the PHO boxes present in E. coli phosphate-regulated promoters. The R. meliloti ndvF mutants grow poorly at a phosphate concentration of 2 mM, and we hypothesize that their symbiotic phenotype results from their failure to grow during the nodule infection process. Presumably, the PhoCDET transport system is employed by the bacteria in the soil environment, where the concentration of available phosphate is normally 0.1 to 1 microM.
Insights
Rhizobium meliloti uses the phoCDET genes for phosphate uptake, crucial for nodule invasion and nitrogen fixation in alfalfa. This ABC transport system is vital for bacterial growth in low-phosphate soil environments.
Area of Science:
- Microbiology
- Plant-Bacterial Interactions
- Molecular Biology
Background:
- Rhizobium meliloti forms nitrogen-fixing root nodules on alfalfa.
- The ndvF locus on the pEXO megaplasmid is essential for nodule invasion and N2 fixation.
Purpose of the Study:
- To identify and characterize the genes within the ndvF locus.
- To elucidate the function of the ndvF locus in Rhizobium meliloti's symbiotic relationship with alfalfa.
Main Methods:
- Gene identification and sequencing of the ndvF locus.
- Homology analysis of encoded proteins with known transport systems.
- Gene expression analysis under phosphate-limiting conditions.
- Phenotypic analysis of ndvF mutants under varying phosphate concentrations.
Main Results:
- The ndvF locus comprises four genes, phoCDET, encoding an ABC-type phosphate (Pi) transport system.
- PhoC and PhoD proteins show homology to E. coli phosphonate transporters PhnC and PhnD.
- PhoT and PhoE proteins are homologous to E. coli PhnE.
- phoD and phoE genes are induced by phosphate starvation.
- The phoC promoter contains PHO box-like elements, suggesting phosphate regulation.
- ndvF mutants exhibit poor growth at 2 mM phosphate.
Conclusions:
- The phoCDET genes constitute a phosphate uptake system in R. meliloti.
- This transport system is likely essential for bacterial survival and symbiotic function in low-phosphate soil.
- Impaired phosphate uptake may explain the symbiotic defect observed in ndvF mutants during nodule infection.