Engineering integrative vectors based on phage site-specific recombination mechanism for Lactococcus lactis

Site-specific integration system allows foreign DNA to be integrated into the specific site of the host genome, enabling stable expression of heterologous protein. In this study, integrative vectors for secretion and surface display of proteins were constructed based on a lactococcal phage TP901-1 integrating system.

The constructed integration system comprises of a lactococcal promoter (P_nisA or P₁₇₀), phage attachment site (attP) from bacteriophage TP901-1, a signal peptide (USP45 or SPK1) for translocation of the target protein, and a PrtP₃₄₄ anchor domain in the case of the integrative vectors for surface display. There were eight successfully constructed integrative vectors with each having a different combination of promoter and signal peptide; pS1, pS2, pS3 and pS4 for secretion, and pSD1, pSD2, pSD3 and pSD4 for surface display of desired protein. The integration of the vectors into the host genome was assisted by a helper vector harbouring the integrase gene. A nuclease gene was used as a reporter and was successfully integrated into the L. lactis genome and Nuc was secreted or displayed as expected. The signal peptide SPK1 was observed to be superior to USP45-LEISSTCDA fusion in the secretion of Nuc. As for the surface display integrative vector, all systems developed were comparable with the exception of the combination of P₁₇₀ promoter with USP45 signal peptide which gave very low signals in whole cell ELISA.

The engineered synthetic integrative vectors have the potential to be used for secretion or surface display of heterologous protein production in lactococcal expression system for research or industrial purposes, especially in live vaccine delivery.