Enhancing the Production\nof ε‑Poly‑l‑Lysine by Engineering\nthe Sucrose Metabolism Pathway\nin <i>Streptomyces albulus</i> PD‑1 Using Cane Molasses

Cane molasses, a sugar-rich agro-industrial byproduct,\nwas used\nto enhance the production of ε-poly-l-lysine (ε-PL)\nin <i>Streptomyces albulus</i> PD-1 as a cost-effective\ncarbon source. The sucrose metabolism pathway was engineered by heterologously\nexpressing sucrose-6-phosphate hydrolase from <i>Escherichia\ncoli</i> W. The optimization of various promoters identified\nthe SP44 promoter, increasing the total sugar utilization rate by\n2.76-fold compared with the <i>ermEp*</i> promoter. Additionally,\nadaptive laboratory evolution improved the total sugar utilization\nrate. The evolved strain achieved an ε-PL titer of 2.65 ±\n0.15 g/L in flask experiments, increasing the ε-PL titer by\n7.16-fold compared with the unevolved strain. Comparative transcriptomic\nanalyses revealed that the enhanced tolerance of the evolved strain\nto high concentrations of cane molasses was primarily due to modifications\nin the sucrose metabolism pathways, microbial metabolism in heavy\nmetals and phenols, and the amino acids transport and metabolic pathways.\nThese changes enabled more efficient ε-PL production. During\nfed-batch fermentation in a 5-L fermentor using a concentration of\n50 g/L cane molasses, the ε-PL titer reached 36.88 ± 0.62\ng/L, and dry cell weight was 41.1 ± 1.0 g/L. This study illustrates\nthat cane molasses is an economical carbon source for producing ε-PL\non an industrial scale.