Although numerous bacteria possess genes annotated iol in their genomes, there have been very few studies on the possibly associated myo-inositol metabolism and its significance for the cell. We found that Corynebacterium glutamicum utilizes myo-inositol as a carbon and energy source, enabling proliferation with a high maximum rate of 0.35 h ؊1 . Whole-genome DNA microarray analysis revealed that 31 genes respond to myo-inositol utilization, with 21 of them being localized in two clusters of >14 kb. A set of genomic mutations and functional studies yielded the result that some genes in the two clusters are redundant, and only cluster I is necessary for catabolizing the polyol. There are three genes which encode carriers belonging to the major facilitator superfamily and which exhibit a >12-fold increased mRNA level on myo-inositol. As revealed by mutant characterizations, one carrier is not involved in myo-inositol uptake whereas the other two are active and can completely replace each other with apparent K m s for myo-inositol as a substrate of 0.20 mM and 0.45 mM, respectively. Interestingly, upon utilization of myo-inositol, the L-lysine yield is 0.10 mol/mol, as opposed to 0.30 mol/mol, with glucose as the substrate. This is probably not only due to myo-inositol metabolism alone since a mixture of 187 mM glucose and 17 mM myo-inositol, where the polyol only contributes 8% of the total carbon, reduced the L-lysine yield by 29%. Moreover, genome comparisons with other bacteria highlight the core genes required for growth on myo-inositol, whose metabolism is still weakly defined.Inositol is a building block of plants and is thus probably one of the sources of traces of myo-inositol, or its phosphorylated derivative myo-inositol hexakisphosphate, in soil (26). Accordingly, there are indications that a number of microorganisms are able to utilize myo-inositol. For instance, the soil-inhabiting Rhizobiaceae family members Sinorhizobium fredii (16) and Rhizobium leguminosarum (9) have the ability to catabolize or even grow on myo-inositol and this feature may increase their fitness for better nodulating the host plant (10). Also, Klebsiella (Aerobacter) aerogenes is able to utilize myo-inositol (19) and early biochemical work with this organism established how the polyol could be metabolized (2) (Fig. 1).As can be seen from their genome sequences, a large number of bacteria have genes which are annotated as iol genes. These are often clustered, for example, the iolDEB genes in R. leguminosarum, which are required for growth on inositol (9), or in Clostridium perfringens, where a cluster of 13 genes is induced by myo-inositol with the participation of the regulator IolR (17). In Bacillus subtilis, there is an iol divergon comprising iolABCDEFGHIJ and iolRS whose repression by glucose is in part CcpA dependent (29,30). Relatively few studies have been done to demonstrate the participation of the iol genes in inositol metabolism, and there are a very limited number of biochemical studies on their enzyme function. A my...

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. glutamicum Delta ptsF 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.

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