Figure 1. The metabolic pathway from xylose to BT.

Xylose is oxidized to xylonolactone by the xylose dehydrogenase (encoded by CcxylB). Xylonolactone is hydrolyzed to xylonate by the xylonolactonase (encoded by xylC). Xylonate dehydratase (encoded by yjhG) catalyzes the dehydration of xylonate to produce 3-deoxy-pentulosonate. Then 3-deoxy-pentulosonate is decarboxylated by 2-keto acid decarboxylase (encoded by mdlC) to form 3,4-dihydroxybutyraldehyde. At last, 3,4-dihydroxybutyraldehyde is reduced by the aldehyde reductase (encoded by adhP) to generate BT. Xylose isomerase (encoded by xylA) and xylulose kinase (encoded by EcxylB) responsible for xylose metabolism are knocked out to block the branched pathways.