Secretion of TEM beta-lactamase with signal sequences isolated from the chromosome of Lactococcus lactis subsp. lactis

Abstract

With TEM beta-lactamase as a reporter gene, a set of expression-secretion-promoting fragments were isolated from the chromosome of Lactococcus lactis subsp. lactis. The fact that only translocated beta-lactamase renders cells resistant to ampicillin allowed direct ampicillin selection with an Escherichia coli vector (pKTH33). The clones showing the greatest ampicillin resistance were subcloned onto a replicon capable of replication in lactic acid bacteria (pVS2), and the nucleotide sequences of the relevant fragments were determined. The structure of the secretion-promoting fragments in general resembled that of gram-positive true signal sequences, with a strongly positively charged N terminus, a long hydrophobic core, and a putative signal peptidase recognition site. The promoterlike sequences preceding the signal sequences matched well with those of previously published lactococcal promoters. In addition to E. coli, the functioning of these expression-secretion cassettes was studied in three gram-positive hosts: Bacillus subtilis, L. lactis, and Lactobacillus plantarum. Efficient expression and secretion of TEM beta-lactamase into the culture medium of each gram-positive host was obtained. Furthermore, when a strain of L. lactis subsp. lactis showing increased sensitivity to lysozyme was compared with a standard laboratory strain, threefold-higher secreted enzyme activities were detected.