VanX, a bacterial D-alanyl-D-alanine dipeptidase: resistance, immunity, or survival function?

The zinc-containing D-alanyl-D-alanine (D-Ala-D-Ala) dipeptidase VanX has been detected in both Gram-positive and Gram-negative bacteria, where it appears to have adapted to at least three distinct physiological roles. In pathogenic vancomycin-resistant enterococci, vanX is part of a five-gene cluster that is switched on to reprogram cell-wall biosynthesis to produce peptidoglycan chain precursors terminating in D-alanyl-D-lactate (D-Ala-D-lactate) rather than D-Ala-D-Ala. The modified peptidoglycan exhibits a 1, 000-fold decrease in affinity for vancomycin, accounting for the observed phenotypic resistance. In the glycopeptide antibiotic producers Streptomyces toyocaensis and Amylocatopsis orientalis, a vanHAX operon may have coevolved with antibiotic biosynthesis genes to provide immunity by reprogramming cell-wall termini to D-Ala-D-lactate as antibiotic biosynthesis is initiated. In the Gram-negative bacterium Escherichia coli, which is never challenged by the glycopeptide antibiotics because they cannot penetrate the outer membrane permeability barrier, the vanX homologue (ddpX) is cotranscribed with a putative dipeptide transport system (ddpABCDF) in stationary phase by the transcription factor RpoS (sigma(s)). The combined action of DdpX and the permease would permit hydrolysis of D-Ala-D-Ala transported back into the cytoplasm from the periplasm as cell-wall crosslinks are refashioned. The D-Ala product could then be oxidized as an energy source for cell survival under starvation conditions.