High expression of toxic Streptomyces phospholipase D in Escherichia coli under salt stress and its mechanism

Phospholipase D (PLD) cytotoxicity limits its high heterologous expression and further application in the food industry. The specific physiological consequences of highly expressed PLD in vivo are unclear. In this study, we observed that PLD expression by Escherichia coli caused phosphatidic acid accumulation and membrane permeability enhancement, characterized by the disruption of ionic homeostasis with enhanced K+ efflux and Na+ influx, which eventually may result in cell injury or death. Moreover, we found that cations (Na+, K+, Li+, or Mg2+) stress alleviated cell growth inhibition and profoundly increased PLD production. A large quantity of PLD (1.1 × 106 U/L) was obtained in a bioreactor. The mechanism for Na+/K+‐induced protection may be the improvement of oxidative phosphorylation through positive changes in membrane potential via redistribution of Na+/K+ inside and outside the cell, which was accompanied by enhanced Na+ efflux. Combined, these findings provided novel insights for PLD function and PLD production.