Reconstituting Spore Cortex Peptidoglycan Biosynthesis Reveals a Deacetylase That Catalyzes Transamidation

Some bacteria survive in nutrient-poor environments and resist killing by antimicrobials by forming spores. The cortex layer of the peptidoglycan cell wall that surrounds mature spores contains a unique modification, muramic-δ-lactam, that is essential for spore germination and outgrowth. Two proteins, the amidase CwlD and the deacetylase PdaA, are required for muramic-δ-lactam synthesis in cells, but their combined ability to generate muramic-δ-lactam has not been directly demonstrated. Here we report an in vitro reconstitution of cortex peptidoglycan biosynthesis, and we show that CwlD and PdaA together are sufficient for muramic-δ-lactam formation. Our method enables characterization of the individual reaction steps, and we show for the first time that PdaA has transamidase activity, catalyzing both the deacetylation of N-acetylmuramic acid and cyclization of the product to form muramic-δ-lactam. This activity is unique among peptidoglycan deacetylases and is notable because it may involve the direct ligation of a carboxylic acid with a primary amine. Our reconstitution products are nearly identical to the cortex peptidoglycan found in spores, and we expect that they will be useful substrates for future studies of enzymes that act on the spore cortex.