PDZ-domain containing proteases,also named HtrA family proteases,are a group of important signaling proteins in both eukaryotic and prokaryotic cells.These proteases usually elicit signaling processes by an irreversible,proteolytic manner.How to identify the substrates of these proteases turns out to be a quite difficult problem.Here we found that a PDZ-domain protease,Prc,involved in regulating virulence and osmotic stress of a gram-negative bacterium Xanthomonas campestris pv.campestris.