Conversion of L-arabinose to L-ribose by genetically engineered Candida tropicalis

L-Ribose, a starting material for the synthesis of L-nucleoside, has attracted lots of attention since L-nucleoside is responsible for the antiviral activities of the racemic mixtures of nucleoside enantiomers. In this study, the L-ribulose-producing Candida tropicalis strain was engineered for the conversion of L-arabinose to L-ribose. For the construction of a uracil auxotroph, the URA3 gene was excised by homologous recombination. The expression cassette of codon-optimized L-ribose isomerase gene from Acinetobacter calcoaceticus DL-28 under the control of the GAPDH promoter was integrated to the uracil auxotroph. The resulting strain, K1 CoSTP2 LsaAraA AcLRI, was cultivated with the glucose/L-arabinose mixture. At 45.5 h of fermentation, 6.0 g/L of L-ribose and 3.2 g/L of L-ribulose were produced from 30 g/L of L-arabinose. The proportion between L-ribose and L-ribulose was approximately 2:1 and the conversion yield of L-arabinose to L-ribose was about 20% (w/w). The L-ribose-producing yeast strain was successfully constructed for the first time and could convert L-arabinose to L-ribose in one-pot fermentation using the mixture of glucose and L-arabinose.