Regulation of the phosphate stress response in Rhizobium meliloti by phoB

Alkaline phosphatase activity and phosphate transport rates in Rhizobium meliloti increased significantly when medium phosphate levels decreased tO approximately 10 μM. Both responses were abolished in a Tn5::phoB mutant, but the mutant could be complemented by a plasmid that contained cloned R. meliloti phoB. The PhoB^- mutant had a normal symbiosis phenotype under growth conditions that supplied either limiting or nonlimiting levels of phosphate to the host plant Medicago sativa, suggesting that induction of genes by PhoB was not required for normal symbiotic function.