Restructured Lactococcus lactis strains with emergent properties constructed by a novel highly efficient screening system

BACKGROUND: After 2.83% genome reduction in Lactococcus lactis NZ9000, a good candidate host for proteins production was obtained in our previous work. However, the gene deletion process was time consuming and laborious. Here, we proposed a convenient gene deletion method suitable for large-scale genome reduction in L. lactis NZ9000. RESULTS: Plasmid pNZ5417 containing a visually selectable marker P(nisZ)-lacZ was constructed, which allowed more efficient and convenient screening of gene deletion mutants. Using this plasmid, two large nonessential DNA regions, L-4A and L-5A, accounting for 1.25% of the chromosome were deleted stepwise in L. lactis 9k-3. When compared with the parent strain, the mutant L. lactis 9k-5A showed better growth characteristics, transformability, carbon metabolic capacity, and amino acids biosynthesis. CONCLUSIONS: Thus, this study provides a convenient and efficient system for large-scale genome deletion in L. lactis through application of visually selectable marker, which could be helpful for rapid genome streamlining and generation of restructured L. lactis strains that can be used as cell factories.