Nodulating ability of Rhizobium tropici is conditioned by a plasmid-encoded citrate synthase.
Rhizobium species elicit the formation of nitrogen-fixing root nodules through a complex interaction between bacteria and plants. Various bacterial genes involved in the nodulation and nitrogen-fixation processes have been described and most have been localized on the symbiotic plasmids (pSym). We have found a gene encoding citrate synthase on the pSym plasmid of Rhizobium tropici, a species that forms nitrogen-fixing nodules on the roots of beans (Phaseolus vulgaris) and trees (Leucaena spp.). Citrate synthase is a key metabolic enzyme that incorporates carbon into the tricarboxylic acid cycle by catalysing the condensation of acetyl-CoA and oxaloacetic acid to form citrate. R. tropici pcsA (the plasmid citrate synthase gene) is closely related to the corresponding genes of Proteobacteria. pcsA inactivation by a Tn5-mob insertion causes the bacteria to form fewer nodules (30-50% of the original strain) and to have a decreased citrate synthase activity in minimal medium with sucrose. A clone carrying the pcsA gene complemented all the phenotypic alterations of the pcsA mutant, and conferred Rhizobium leguminosarum bv. phaseoli (which naturally lacks a plasmid citrate synthase gene) a higher nodulation and growth capacity in correlation with a higher citrate synthase activity. We have also found that pcsA gene expression is sensitive to iron availability, suggesting a possible role of pcsA in iron uptake.
Pardo
M A
MA
Departamento de Genética Molecular, Centro de Investigación sobre Fijación de Nitrógeno, UNAM, México.
Lagunez
J
J
Miranda
J
J
Martínez
E
E
D016960:Agrobacterium tumefaciens / Q000235:genetics; D000595:Amino Acid Sequence; D001483:Base Sequence; D002950:Citrate (si)-Synthase / Q000235:genetics* / Q000378:metabolism*; D002952:Citric Acid Cycle; D003001:Cloning, Molecular; D003227:Conjugation, Genetic; D007887:Fabaceae / Q000382:microbiology; D005798:Genes, Bacterial; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D009586:Nitrogen Fixation / Q000235:genetics*; D010802:Phylogeny; D010946:Plants, Medicinal; D010957:Plasmids; D015183:Restriction Mapping; D012231:Rhizobium / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism*; D014197:Trees / Q000382:microbiology
D002950:Citrate (si)-Synthase