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

Abstract

Edwardsiella tarda is a severe aquaculture pathogen with a broad host range that includes humans, animal, and fish. A gene ( degP Et ) encoding a DegP homologue was cloned from TX01, a pathogenic E. tarda strain isolated from diseased fish. DegP Et shares high sequence identities with the DegP proteins of several bacterial species. Functional analyses showed that degP Et could complement the temperature-sensitive phenotype of an Escherichia coli degP null mutant. Expression of degP Et in TX01 was modulated by growth phase and temperature, the latter possibly through the action of the σ E-like factor. Overexpression of degP Et (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 DegP Et purified from E. coli was a serine protease that exhibited maximum activity at 40 °C and pH8.0. The proteolytic activity of recombinant DegP Et depended on the catalytic triad and the PDZ domains. Immunoprotective analyses showed that purified recombinant DegP Et was a protective immunogen that could induce the production of specific serum antibodies and elicit strong protective immunity in fish vaccinated with DegP Et.