Structural features of the lac promoter affecting gusA expression in Lactobacillus casei

With the aim of designing efficient expression systems for Lactobacillus casei, different factors affecting gene expression from the strong and highly modulated lac promoter have been systematically analyzed, by using the Escherichia coli beta-glucuronidase gene (gusA) as reporter. The activity of this enzyme (GUS) was quantified when plac:: gusA fusions were cloned in plasmids with different copy number or when gusA was inserted in the chromosomal lactose operon (single copy). Results showed a clear gene dosage effect and a positive influence of the native lac operon transcription and translation signals on GUS expression.