The Bradyrhizobium japonicum phoB gene is required for phosphate-limited growth but not for symbiotic nitrogen fixation.

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We identified by cloning and DNA sequence analysis the phosphate regulatory gene phoB of Bradyrhizobium japonicum. The deduced gene product displayed pronounced similarity to the PhoB protein of Sinorhizobium meliloti (71.4% identical amino acids). Escherichia coli (50.2%) and other bacterial species. Insertion of a kanamycin resistance cassette into phoB led to impaired growth of the B. japonicum mutant in media containing approximately 25 microM phosphate or less. A standard plant infection test using wild-type and phoB-defective B. japonicum strains showed that the phoB mutation had no effect on the symbiotic properties of B. japonicum with its soybean host plant.