Abstract
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 CO 2 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%.
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