Acid or erythromycin stress significantly improves transformation efficiency through regulating expression of DNA binding proteins in Lactococcus lactis F44

Lactococcus lactis is a gram-positive bacterium used extensively in the dairy industry and food fermentation, and its biological characteristics are usually improved through genetic manipulation. However, poor transformation efficiency was the main restriction factor for the construction of engineered strains. In this study, the transformation efficiency of L. lactis F44 showed a 56.1-fold increase in acid condition (pH 5.0); meanwhile, erythromycin stress (0.04 mu g/mL) promoted the transformation efficiency more significantly (76.9-fold). Notably, the transformation efficiency of F44e (L. lactis F44 harboring empty pLEB124) increased up to 149.1-fold under the synergistic stresses of acid and erythromycin. In addition, the gene expression of some DNA binding proteins (DprA, RadA, RadC, RecA, RecQ, and SsbA) changed correspondingly. Especially for radA, 25.1-fold improvement was detected when F44e was exposed to pH 5.0. Overexpression of some DNA binding proteins could improve the transformation efficiency. The results suggested that acid or erythromycin stress could improve the transformation efficiency of L. lactis through regulating gene expression of DNA binding proteins. We have proposed a simple but promising strategy for improving the transformation efficiency of L. lactis and other hard-transformed microorganisms.