1.1.1.16 synthesis a yeast strain capable of consuming lactose intracellularly is engineered to produce tagatose from lactose. GAL1 coding for galactose kinase is deleted to eliminate galactose utilization. Heterologous xylose reductase (XR) and galactitol dehydrogenase (GDH) are introduced into the Gal1 deletion strain. The expression levels of XR and GDH are adjusted to maximize tagatose production. The resulting engineered yeast produces 37.69 g/l of tagatose from lactose with a tagatose and galactose ratio of 9:1 in the reaction broth 761971 1.1.1.16 synthesis the enzyme can be used to produce optically pure building blocks and for the bioconversion of bioactive compounds -, 740390