TYPE-III PROTEIN SECRETION SYSTEMS IN BACTERIAL PATHOGENS OF ANIMALS AND PLANTS

Various gram-negative animal and plant pathogens use a novel, sec-inde pendent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion s ystems inject (translocate) proteins into the cytosol of eukaryotic ce lls, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and ot her cellular processes. Accordingly, some type III secretion systems a re activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pat hogenesis phenotypes. In contrast to the secreted virulence factors, m ost of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion appara tus show additional homologies to flagellar biosynthetic proteins, whi le a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones wh ich specifically bind to individual secreted proteins play an importan t role in type III protein secretion, apparently by preventing prematu re interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizont al genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental s timuli by networks of transcription factors. This review comprises a c omparison of the structure, function, regulation, and impact on host c ells of the type III secretion systems in the animal pathogens Yersini a spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimur ium, enteropathogenic Escherichia coli, and Chlamydia spp. and the pla nt pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearu m, Xanthomonas campestris, and Rhizobium spp.