Screening and characterization of secretion signals from Lactococcus lactis ssp. cremoris LM0230

A secretion signal sequence-selection vector (pGS40) was constructed based on an α-amylase gene lacking a secretion signal and employed for selecting secretion signals from Lactococcus lactis ssp. cremoris LM0230 chromosomal DNA. Six fragments were identified based on their ability to restore α-amylase secretion in E. coli, and among these, a fragment, S405, conferred the highest secretion activity (84%) in E. coli. Meanwhile, S407, which conferred poor secretion activity in E. coli, was quite active in L. lactis. The results suggested that the efficiency of a secretion signal depended on the host. All six fragments had an open reading frame (ORF) fused to the reporter gene, and the potential Shine-Dalgarno (SD) sequence and putative promoter sequences were located upstream of the ORF. Deduced amino acid sequences from the six fragments did not show any homology with known secretion signals. However, they contained three distinguished structural features and cleavage sites, commonly found among typical secretion signals. The characterized secretion signals could be useful for the construction of food-grade secretion vectors and gene expression in LAB.