Use of the Escherichia coli beta-glucuronidase (gusA) gene as a reporter gene for analyzing promoters in lactic acid bacteria.

A transcriptional fusion vector, designated pNZ272, based on the promoterless beta-glucuronidase gene (gusA) of Escherichia coli as a reporter gene, has been constructed for lactic acid bacteria. The replicon of pNZ272 was derived from the Lactococcus lactis plasmid pSH71, allowing replication in a wide range of gram-positive bacteria and E. coli. The applicability of pNZ272 and the expression of the gusA gene in L. lactis was demonstrated in shotgun cloning experiments with lactococcal chromosomal and bacteriophage DNA. In addition, three defined lactococcal promoters were inserted in pNZ272: the plasmid-derived lacA promoter, the chromosomal usp45 promoter, and a promoter from bacteriophage phi SK11G. The three resulting plasmids showed beta-glucuronidase activity in a gusA-deficient E. coli strain and in four species of lactic acid bacteria belonging to the genera Lactobacillus, Lactococcus, and Leuconostoc. The copy numbers of the gusA-expressing plasmids were similar within a single species of lactic acid bacteria. However, the specific beta-glucuronidase activity and the gusA mRNA levels varied considerably both within a single species and among different species of lactic acid bacteria. The transcriptional start site of all three promoters was determined and found to be identical in the different species. The results of this comparative promoter analysis indicate that the requirements for efficient transcription initiation differ among the lactic acid bacteria studied.