Analysis of Edwardsiella tarda DegP, a serine protease and a protective immunogen

Edwardsiella tarda is a severe aquaculture pathogen with a broad host range that includes humans, animal, and fish. A gene (degPEt) encoding a DegP homologue was cloned from TX01, a pathogenic E. tarda strain isolated from diseased fish. DegPEt shares high sequence identities with the DegP proteins of several bacterial species. Functional analyses showed that degPEt could complement the temperature-sensitive phenotype of an Escherichia coli degP null mutant. Expression of degPEt in TX01 was modulated by growth phase and temperature, the latter possibly through the action of the σE-like factor. Overexpression of degPEt (i) enhanced the ability of TX01 to disseminate in fish blood at the advanced stage of infection, (ii) heightened the activity of type 2 autoinducer, and (iii) increased the expression of luxS and the genes encoding components of the virulence-associated type III secretion system. Recombinant DegPEt purified from E. coli was a serine protease that exhibited maximum activity at 40 °C and pH8.0. The proteolytic activity of recombinant DegPEt depended on the catalytic triad and the PDZ domains. Immunoprotective analyses showed that purified recombinant DegPEt was a protective immunogen that could induce the production of specific serum antibodies and elicit strong protective immunity in fish vaccinated with DegPEt.