Myo-inositol facilitators IolT1 and IolT2 enhance d-mannitol formation from d-fructose in Corynebacterium glutamicum

Reduction of d-fructose to d-mannitol by whole-cell biotransformation with recombinant resting cells of Corynebacterium glutamicum ATCC13032 requires the coexpression of mdh and fdh, which encode mannitol and formate dehydrogenases, respectively. However, d-mannitol formation is limited by the uptake of d-fructose in its unphosphorylated form, because additional expression of the sugar facilitator from Zymomonas mobilis resulted in a significantly increased productivity. Here we identified similarities of the myo-inositol transporters IolT1 and IolT2 of C. glutamicum to the sugar facilitator of Z. mobilis. The myo-inositol transporter genes were both individually overexpressed and deleted in recombinants expressing mdh and fdh. Biotransformation experiments showed that the presence and absence, respectively, of IolT1 and IolT2 significantly influenced d-mannitol formation, indicating a d-fructose transport capability of these transporters. For further evidence, a C. glutamicumΔptsF mutant unable to grow with d-fructose was complemented with a heterologous fructokinase gene. This resulted in restoration of growth with d-fructose. Using overexpressed iolT1, mdh and fdh, d-mannitol formation obtained with C. glutamicum was 34.2 g L⁻¹, as opposed to 16 g L⁻¹ formed by the strain overexpressing only mdh and fdh, showing the suitability of myo-inositol transporters for d-fructose uptake to obtain d-mannitol formation by whole-cell biotransformation with C. glutamicum.