In this work we present modifications that improve the performance of the previously described Pm-nasF NahR-XylS regulatory cascade expression system at two levels: (i) the regulatory module has been modified to include nasR under Psal control and a constitutively expressed gfp to allow bacterial infection to be followed; (ii) the expression module has been placed in high and low-copy number plasmids, which were modified to obtain more versatile expression vectors. This evidence concerns the gene PRB1 and bacterial infectious disease.