Dual Antibiotic-Free Plasmid Systems Enable High-Efficiency l-Fucose Biosynthesis

l-Fucose, a functional monosaccharide with significant commercial potential in the pharmaceutical, nutraceutical, and cosmetic industries, faces challenges in microbial production due to antibiotic-dependent plasmid maintenance systems. This study presents a dual antibiotic-free plasmid strategy in engineered Escherichia coli BL21(DE3) to achieve high-efficiency l-fucose biosynthesis. By integration of the hok/sok toxin-antitoxin system and a cysC-based auxotrophic selection into two plasmids, genetic stability and plasmid retention were ensured without antibiotics. Metabolic pathway optimization involved enhancing GDP-l-fucose supply via promoter replacements, genomic integration of key enzymes (α1,2-fucosyltransferase and α-l-fucosidase), and blocking l-fucose degradation. The engineered strain demonstrated robust performance, producing 7.99 g/L of l-fucose in shake-flask fermentation and 61.91 g/L via fed-batch cultivation─both antibiotic-free. This titer represents the highest reported l-fucose yield to date, highlighting the effectiveness of combining toxin-antitoxin and auxotrophic systems for sustainable, high-productivity microbial manufacturing.