Identification of two scyllo-inositol dehydrogenases in Bacillus subtilis

scyllo-Inositol (SI) is a stereoisomer of inositol whose catabolism has not been characterized in bacteria. We found that Bacillus subtilis 168 was able to grow using SI as its sole carbon source and that this growth was dependent on a functional iol operon for catabolism of myo-inositol (MI; another inositol isomer, which is abundant in nature). Previous studies elucidated the MI catabolic pathway in B. subtilis as comprising multiple stepwise reactions catalysed by a series of Iol enzymes. The first step of the pathway converts MI to scyllo-inosose (SIS) and involves the MI dehydrogenase IolG. Since IolG does not act on SI, we suspected that there could be another enzyme converting SI into SIS, namely an SI dehydrogenase. Within the whole genome, seven genes paralogous to iolG have been identified and two of these, iolX and iolW (formerly known as yisS and yvaA, respectively), were selected as candidate genes for the putative SI dehydrogenase since they were both prominently expressed when B. subtilis was grown on medium containing SI. iolX and iolW were cloned in Escherichia coli and both were shown to encode a functional enzyme, revealing the two distinct SI dehydrogenases in B. subtilis. Since inactivation of iolX impaired growth with SI as the carbon source, IolX was identified as a catabolic enzyme required for SI catabolism and it was shown to be NAD(+) dependent. The physiological role of IolW remains unclear, but it may be capable of producing SI from SIS with NADPH oxidation.