Production and purification of the penicillin-binding protein 3 from Pseudomonas aeruginosa.

The penicillin-binding proteins (PBPs) are peripheral membrane enzymes that catalyze the final steps for the biosynthesis of the essential bacterial cell wall heteropolymer peptidoglycan. Bacteria produce a number and variety of PBPs which are classified as either high molecular weight or low molecular weight PBPs. The high molecular weight PBPs are multimodular being comprised of an N-terminal membrane anchor followed by a non-penicillin binding domain and a C-terminal penicillin-binding domain. The penicillin-binding domain functions as a serine-acyl transpeptidase to catalyze the crosslinking of neighboring glycan strands within the peptidoglycan sacculus. PBP 3 from Escherichia coli has been studied extensively and it has been shown to be responsible for the synthesis of peptidoglycan during the division and septation of the cells. The opportunistic human pathogen Pseudomonas aeruginosa produces a similar compliment of PBPs to E. coli, but differences in their organization and function have been noted. To investigate these differences further, appropriate quantities of each of the P. aeruginosa PBPs are required in forms amenable for study both in vivo and in vitro. Herein, we describe the cloning and expression of the ftsI gene encoding PBP 3from P. aeruginosa. The PBP was engineered in soluble form to facilitate its study in vitro and with a hexa-His tag to permit its facile purification by affinity chromatography. The recombinant proteins were demonstrated to bind penicillin and these forms of the PBPs were shown to be useful in studying their localization within their host cells by immunogold transmission electron microscopy.

Function of penicillin-binding protein 2 in viability and morphology of Pseudomonas aeruginosa.

OBJECTIVES: To investigate the function of penicillin-binding protein 2 (PBP 2) in Pseudomonas aeruginosa PAO1. METHODS: The growth and morphology of P. aeruginosa cultured in the absence and presence of mecillinam was assessed. The gene encoding PBP 2, pbpA, was identified in the genome of P. aeruginosa PAO1 and both its full-length and an engineered truncated form were cloned and expressed in Escherichia coli. Site-directed mutagenesis was used to confirm Ser-327 as the catalytic nucleophile of its transpeptidase domain. Allelic exchange was used to construct a chromosomal mutant of pbpA in strain PAO1. RESULTS: PAO1 grew with a spherical morphology in the presence of mecillinam at concentrations as high as 2000 mg/L. Both wild-type and truncated, soluble forms of PBP 2 were shown to bind penicillins and a competition assay demonstrated their specificity for mecillinam. The PAO1 DeltapbpA insertional mutant also grew as spheres, and complementation with a plasmid encoding active pbpA, but not with an inactive Ser-327 --> Ala derivative, restored rod-shape morphology. MIC values of a variety of beta-lactams were significantly lower for the insertional mutant compared with wild-type PAO1. The muropeptide profile of peptidoglycan from PAO1 DeltapbpA analysed by HPLC/MALDI TOF MS indicated wild-type levels of cross-linking despite the loss of PBP 2 transpeptidase activity. CONCLUSIONS: PBP 2 in P. aeruginosa is responsible for the rod-shape morphology of the cells and contributes significantly to beta-lactam resistance. The viability of cells lacking an active PBP 2 suggests that the organization of the peptidoglycan biosynthetic machinery is different in this pathogen compared with E. coli.