Mannitol‐1‐phosphate dehydrogenases/phosphatases: a family of novel bifunctional enzymes for bacterial adaptation to osmotic stress

The nutritionally versatile soil bacterium Acinetobacter baylyi ADP1 copes with salt stress by the accumulation of compatible solutes, a strategy that is widespread in nature. This bacterium synthesizes the sugar alcohol mannitol de novo in response to osmotic stress. In a previous study, we identified MtlD, a mannitol‐1‐phosphate dehydrogenase, which is essential for mannitol biosynthesis and which catalyses the first step in mannitol biosynthesis, the reduction of fructose‐6‐phosphate (F‐6‐P) to the intermediate mannitol‐1‐phosphate (Mtl‐1‐P). Until now, the identity of the second enzyme, the phosphatase that catalyses the dephosphorylation of Mtl‐1‐P to mannitol, was elusive. Here we show that MtlD has a unique sequence among known mannitol‐1‐phosphate dehydrogenases with a haloacid dehalogenase (HAD)‐like phosphatase domain at the N‐terminus. This domain is indeed shown to have a phosphatase activity. Phosphatase activity is strictly Mg²⁺ dependent. Nuclear magnetic resonance analysis revealed that purified MtlD catalyses not only reduction of F‐6‐P but also dephosphorylation of Mtl‐1‐P. MtlD of A. baylyi is the first bifunctional enzyme of mannitol biosynthesis that combines Mtl‐1‐P dehydrogenase and phosphatase activities in a single polypeptide chain. Bioinformatic analysis revealed that the bifunctional enzyme is widespread among Acinetobacter strains but only rarely present in other phylogenetic tribes.