Plasmid curing resulted in improved heterologous gene expression in Leuconostoc citreum EFEL2700

Leuconostoc citreum EFEL2700 isolated from kimchi was used as a host strain for genetic and metabolic engineering in our previous studies, but the cells of EFEL2700 contained a cryptic plasmid (P-cells). Thus, we created plasmid-free cells (F-cells) using the CRISPR/Cas9 system. In this study, we compared the microbial characteristics of P- and F-cells in terms of growth rate, biochemical properties, transformation efficiency, plasmid copy number and protein expression level. When the growth rate was measured in MRS medium at 30°C, no significant difference (P > 0·01) was observed. Biochemical properties, tested using an API 50CHL kit, showed no differences. Transformation efficiency of F-cells, measured using pCB4270, was higher (1·3 × 10⁴ CFU per μg DNA) than that of P-cells (5·0 × 10³ CFU per μg DNA). Copy number after transformation of pCBBgl was 4-fold higher for F-cells than for P-cells. When β-glucosidase activity was assayed in the above experiment, F-cells showed 3·4-fold higher values than P-cells. In conclusion, this study demonstrates that plasmid curing in L. citreum EFEL2700 improves its characteristics as a gene expression host. Significance and Impact of the Study: Leuconostoc citreum EFEL2700 (P-cell) isolated from kimchi is a useful food-grade host for expressing heterologous genes. The presence of a cryptic plasmid is thought to limit efficient gene expression. In this study, we compared the microbial and genetic changes after plasmid curing in this strain. The plasmid-free strain showed improved levels of transformation efficiency, copy number and heterologous gene expression without alterations in phenotypes such as the growth rates and biochemical properties. The resulting strain of L. citreum EFEL2701 (F-cell) can be used as an efficient host for genetic engineering.