hemX is required for production of 2‐ketogluconate, the predominant organic anion required for inorganic phosphate solubilization by B urkholderia sp. H a185

Summary The bacterium B urkholderia sp. H a185 readily solubilizes inorganic phosphate by releasing the low molecular weight organic anion, 2‐ketogluconate. Using random transposon mutagenesis and in silico analysis, a mutation that caused almost complete abolition of phosphate solubilization was located within hemX , which is part of the hem operon. B urkholderia sp. H a185 HemX is a multidomain protein, predicted to encode a bifunctional uroporphyrinogen‐ III synthetase/uroporphyrin‐ III C ‐methyltransferase, which has not previously been implicated in phosphate solubilization. Complementation of hemX restored the ability of the mutant to solubilize phosphate in both plate and liquid cultures. Based on a combination of organic‐anion profiling, quantitative polymerase chain reaction and in silico analyses, hemX was confirmed to be solely responsible for hydroxyapatite solubilization in B urkholderia sp. H a185. It is proposed that the biosynthesis of a yet to be determined redox cofactor by HemX is the main pathway for generating 2‐ketogluconate via a haem‐dependent gluconate 2‐dehydrogenase in B urkholderia sp. H a185.