Engineering the Substrate Transport and Cofactor Regeneration Systems for Enhancing 2′-Fucosyllactose Synthesis in <i>Bacillus subtilis</i>

Abstract

Human milk oligosaccharides (HMOs) have been proven to be beneficial to infants' intestinal health and immune systems. 2'-Fucosyllactose (2'-FL) is the most abundant and thoroughly studied HMO and has been approved to be an additive of infant formula. How to construct efficient and safe microbial cell factories for the production of 2'-FL attracts increasing attention. In this work, we engineered the <i>Bacillus subtilis</i> as an efficient 2'-FL producer by engineering the substrate transport and cofactor guanosine 5'-triphosphate (GTP) regeneration systems. First, we constructed a synthesis pathway for the 2'-FL precursor guanosine 5'-diphosphate-l-fucose (GDP-l-fucose) by introducing the <i>salvage</i> pathway gene <i>fkp</i> from <i>Bacteriodes fragilis</i> and improved the fucose importation by overexpressing the transporters. Then, the complete synthesis pathway of 2'-FL was constructed by introducing the heterologous fucosyltransferases from different sources, and it was found that the gene from <i>Helicobacter pylori</i> was the best one for 2'-FL synthesis. We also improved the substrate lactose importation by introducing heterologous lactose permeases and eliminated endogenous β-galactosidase (<i>yesZ</i>) to block the lactose degradation. Next, the production of 2'-FL and GDP-l-fucose was improved by fine-tuning the expression of cofactor guanosine 5'-triphosphate regeneration module genes <i>gmd</i>, <i>ndk</i>, <i>guaA</i>, <i>guaC</i>, <i>ykfN</i>, <i>deoD</i>, and <i>xpt</i>. Finally, a 3 L fed-batch fermentation was performed, and the highest 2'-FL titer reached 5.01 g/L with a yield up to 0.85 mol/mol fucose. We optimized the synthesis modules of 2'-FL in <i>B. subtilis</i>, and this provides a good starting point for metabolic engineering to further improve 2'-FL production in the future.