Interplay among Membrane-Bound Lytic Transglycosylase D1, the CreBC Two-Component Regulatory System, the AmpNG-AmpD I -NagZ-AmpR Regulatory Circuit, and L1/L2 β-Lactamase Expression in Stenotrophomonas maltophilia

Lytic transglycosylases (LTs) are an important class of enzymes involved in peptidoglycan (PG) cleavage, with the concomitant formation of an intramolecular 1,6-anhydromuramoyl reaction product. There are six annotated LT genes in theStenotrophomonas maltophilia genome, including genes for five membrane-bound LTs (mltA ,mltB1 ,mltB2 ,mltD1 , andmltD2 ) and a gene for soluble LT (slt ). Six LTs ofS. maltophilia KJ were systematically mutated, yielding the ΔmltA , ΔmltB1 , ΔmltB2 , ΔmltD1 , ΔmltD2 , and Δslt mutants. Inactivation ofmltD1 conferred a phenotype of elevated uninduced β-lactamase activity. The underlying mechanism responsible for this phenotype was elucidated by the construction of several mutants and determination of β-lactamase activity. The expression of the genes assayed was assessed by quantitative reverse transcriptase PCR and a promoter transcription fusion assay. The results demonstrate that ΔmltD1 mutant-mediated L1/L2 β-lactamase expression involved thecreBC two-component regulatory system (TCS) and theampNG-ampD I -nagZ-ampR regulatory circuit. The inactivation ofmltD1 resulted inmltB1 andmltD2 upexpression in acreBC - andampNG -dependent manner. The overexpressed MltB1 and MltD2 activity contributed to the expression of the L1/L2 β-lactamase genes via theampNG-ampD I -nagZ-ampR regulatory circuit. These findings reveal, for the first time, a linkage between LTs, the CreBC TCS, theampNG-ampD I -nagZ-ampR regulatory circuit, and L1/L2 β-lactamase expression inS. maltophilia .