A new-generation of Bacillus subtilis cell factory for further elevated scyllo-inositol production

A stereoisomer of inositol, scyllo-inositol (SI), has been regarded as a promising therapeutic agent for Alzheimer's disease. However, this compound is relatively rare, whereas another stereoisomer of inositol, myo-inositol (MI) is abundant in nature. Bacillus subtilis 168 has the ability to metabolize inositol stereoisomers, including MI and SI. Previously, we reported a B. subtilis cell factory with modified inositol metabolism that converts MI into SI in the culture medium. The strain was constructed by deleting all genes related to inositol metabolism and overexpressing key enzymes, IolG and IolW. By using this strain, 10 g/l of MI initially included in the medium was completely converted into SI within 48 h of cultivation in a rich medium containing 2% (w/v) Bacto soytone.

When the initial concentration of MI was increased to 50 g/l, conversion was limited to 15.1 g/l of SI. Therefore, overexpression systems of IolT and PntAB, the main transporter of MI in B. subtilis and the membrane-integral nicotinamide nucleotide transhydrogenase in Escherichia coli respectively, were additionally introduced into the B. subtilis cell factory, but the conversion efficiency hardly improved. We systematically determined the amount of Bacto soytone necessary for ultimate conversion, which was 4% (w/v). As a result, the conversion of SI reached to 27.6 g/l within 48 h of cultivation.

The B. subtilis cell factory was improved to yield a SI production rate of 27.6 g/l/48 h by simultaneous overexpression of IolT and PntAB, and by addition of 4% (w/v) Bacto soytone in the conversion medium. The concentration of SI was increased even in the stationary phase perhaps due to nutrients in the Bacto soytone that contribute to the conversion process. Thus, MI conversion to SI may be further optimized via identification and control of these unknown nutrients.