The regulation of expression of the Lactococcus lactis lactose operon

Translational gene fusions between the Escherichia coli beta-galactosidase (lacZ) gene and the Lactococcus lactis lactose operon were constructed such that transcription from the lactose operon promoter could be assessed by measuring beta-galactosidase activity. The level of beta-galactosidase activity was up to 2.5-fold lower when MG5267 cells, which contain a chromosomal copy of the lactose operon, were grown in glucose compared to those grown in lactose. A greater degree of repression was seen in cells containing the multi-copy plasmid-encoded repressor than in those with only the single-copy chromosomal gene, indicating that the repressor protein is at least partly responsible for the reduction in expression when the cells are grown in glucose (i.e. in the absence of inducer). However, the beta-galactosidase activity was found to be 5.5-fold lower in glucose than in lactose in cells which lacked a fully functional lactose operon. The decrease in expression was shown to be due to glucose repression. The levels of expression when the cells were grown in glucose were considerably higher for MG5267 than for MG1363 suggesting perhaps that a product of the chromosomally-encoded operon in MG5267 has a positive effect on transcription.