Efficiently activated ε-poly-L-lysine production by multiple antibiotic-resistance mutations and acidic pH shock optimization in Streptomyces albulus.

ε-Poly-L-lysine (ε-PL) is a food additive produced by Streptomyces and is widely used in many countries. Working with Streptomyces albulus FEEL-1, we established a method to activate ε-PL synthesis by successive introduction of multiple antibiotic-resistance mutations. Sextuple mutant R6 was finally developed by screening for resistance to six antibiotics and produced 4.41 g/L of ε-PL in a shake flask, which is 2.75-fold higher than the level produced by the parent strain. In a previous study, we constructed a double-resistance mutant, SG-31, with high ε-PL production of 3.83 g/L and 59.50 g/L in a shake flask and 5-L bioreactor, respectively. However, we found that R6 did not show obvious advantages in fed-batch fermentation when compared with SG-31. For further activation of ε-PL synthesis ability, we optimized the fermentation process by using an effective acidic pH shock strategy, by which R6 synthetized 70.3 g/L of ε-PL, 2.79-fold and 1.18-fold greater than that synthetized by FEEL-1 and SG-31, respectively. To the best of our knowledge, this is the highest reported ε-PL production to date. This ε-PL overproduction may be due to the result of R99P and Q856H mutations in ribosomal protein S12 and RNA polymerase, respectively, which may be responsible for the increased transcription of the ε-poly-lysine synthetase gene (pls) and key enzyme activities in the Lys synthesis metabolic pathway. Consequently, ε-PL synthetase activity, intracellular ATP, and Lys concentrations were improved and directly contributed to ε-PL overproduction. This study combined ribosome engineering, high-throughput screening, and targeted strategy optimization to accelerate ε-PL production and probe the fermentation characteristics of hyperyield mutants. The information presented here may be useful for other natural products produced by Streptomyces.