Codon optimization and co-expression of thermostable β-galactosidase and L-arabinose isomerase in lactococcus lactis for single-step production of food-grade D-tagatose

D-Tagatose is a promising low-calorie sucrose substitute having prebiotic properties. In this study, biosynthesis of D-tagatose was achieved by constructing and expressing a polycistronic plasmid encoding codon-optimized thermostable β-galactosidase from Marinomonas sp. BSi20414 and L-arabinose isomerase from Clostridium hylemonae, simultaneously, in Lactococcus lactis NZ3900. Both enzymes were expressed as soluble and functional proteins under control of the nisinA inducible promoter and the optimum concentration of nisin was found to be 1 ng/ml. The induced cells were immobilized in chitosan beads and used for D-tagatose production from lactose as a whole-cell biocatalyst. Results revealed an efficient bioconversion of lactose to D-tagatose and maximum production was accomplished at 60°C after 12 h, starting with 20% (w/ v) lactose. The recombinant cells entrapped in chitosan beads converted up to 34% of lactose into D-tagatose under optimum conditions in a single step that could be implemented in safe-production of food-grade low-calorie sweetener.