Differential regulation of L1 and L2 β-lactamase expression in Stenotrophomonas maltophilia

It has been reported that Stenotrophomonas maltophilia L1 and L2 β-lactamase expression is coordinated. We have isolated S. maltophilia mutants where (i) L1 is constitutively hyper-expressed and L2 is inducible; (ii) L2 is hyper-expressed and L1 is inducible; and (iii) L1 and L2 are constitutively hyper-expressed. The frequency of isolating type 1 and 2 mutants is c. 10−7, indicating that promoter mutations are probably not involved and providing strong evidence that L1 and L2 expression is not, after all, coordinated. The frequency of isolating type 3 mutants is c. 10−9, however, implying that there is a significant overlap between the regulatory mechanisms.