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We report on the biochemical, phylogenetic and genetic regulation of PhoX, the major alkaline phosphatase protein from the soil bacteriumSinorhizobium meliloti. The protein is shown to be a member of a recently identified family of PhoX alkaline phosphatase proteins that is distinct from the well-characterized PhoA family. The matureS. melilotiPhoX protein is located in the periplasm and lacks a 76-amino-acid N-terminal Tat signal peptide. Its phosphatase activity was stimulated by Ca+2 and was optimal at pH 9-11. Except for phytic acid and phosphatidic acid, the enzyme was active against a wide range of phosphorylated substrates (77 nucleotides, phosphorylated carbohydrates and amino acids) and thus exhibited low substrate specificity for C-O-P bonds. No C-P bond substrate was dephosphorylated while the protein was active with two of six phosphoramidate substrates (N-P bond) tested.Sinorhizobium meliloti phoXwas induced when cells were starved for phosphorous and the induction was dependent on the PhoB-regulatory protein. We demonstrate byin vitroanalysis that PhoB protein binds to two tandem 22 nt PhoB binding sites located 64-21 nt upstream from thephoXtranscription start site. Analysis of 95 PhoX orthologues from diverse bacteria revealed two distinct phylogenetic groups of PhoX proteins. The two groups differed in having a conserved glycine (PhoX-I) or asparagine (PhoX-II) next to their putative catalytic Ca+2 binding site. Analysis of thephoXpromoter regions from many of these bacteria also revealed the presence of PhoB binding sites. Alkaline phosphatase proteins of either the PhoX or PhoA family (but rarely both) are found in many bacteria, thus it appears that these are functionally equivalent.