An Unexpected Role for the Periplasmic Phosphatase PhoN in the Salvage of B Vitamers in Salmonella enterica.

Pyridoxal 5'-phosphate (PLP) is the biologically active form of vitamin B, essential for cellular function in all domains of life. In many organisms, such as serovar Typhimurium and , this cofactor can be synthesized or salvaged from B vitamers in the environment. Unexpectedly, strains blocked in PLP biosynthesis were able to use exogenous PLP and pyridoxine 5'-phosphate (PNP) as the source of this required cofactor, while strains of the same genotype could not. Transposon mutagenesis found that was essential for the salvage of PLP and PNP under the conditions tested. encodes a class A nonspecific acid phosphatase (EC 3.1.3.2) that is transcriptionally regulated by the PhoPQ two-component system. The periplasmic location of PhoN was essential for PLP and PNP salvage, and assays confirmed PhoN has phosphatase activity with PLP and PNP as substrates. The data suggest that PhoN dephosphorylates B vitamers, after which they enter the cytoplasm and are phosphorylated by kinases of the canonical PLP salvage pathway. The connection of with PhoPQ and the broad specificity of the gene product suggest is exploiting a moonlighting activity of PhoN for PLP salvage. Nutrient salvage is a strategy used by species across domains of life to conserve energy. Many organisms are unable to synthesize all required metabolites and must rely exclusively on salvage. Others supplement synthesis with the ability to salvage. This study identified an unexpected mechanism present in that allows salvage of phosphorylated B vitamers. and data herein determined that the periplasmic phosphatase PhoN can facilitate the salvage of PLP and PNP. We suggest a mechanistic working model of PhoN-dependent utilization of PLP and PNP and discuss the general role of promiscuous phosphatases and kinases in organismal fitness.