Improving d-mannitol productivity of Escherichia coli: Impact of NAD, CO2 and expression of a putative sugar permease from Leuconostoc pseudomesenteroides

The highly productive whole-cell biotransformation of d-fructose to d-mannitol with recombinant, resting cells of Escherichia coli BL21(DE3) requires the combined expression of mdh, fdh and glf which encode mannitol and formate dehydrogenases and a sugar facilitator, respectively. However, long-term stability of the system was restricted, possibly due to loss of the cofactor NAD, high concentrations of formate, formation of CO2 affecting the internal pH of the cells, accumulation of high intracellular concentrations of d-mannitol, and export of d-mannitol. Downstream of the mdh gene of Leuconostoc pseudomesenteroides, we identified an open reading frame encoding for a putative mannitol permease. The gene was cloned and expressed in E. coli. Biochemical analyses revealed an activity as secondary carrier for d-fructose. Therefore, the carrier was named FupL and participation in d-mannitol transport was excluded. In biotransformation experiments, the productivity of d-mannitol formation obtained with the strain expressing the additional fupL gene was enhanced by 20%.