Type III secretion contributes to the pathogenesis of the soft-rot pathogen Erwinia carotovora: Partial characterization of the hrp gene cluster

The virulence of soft-rot Erwinia species is dependent mainly upon secreted enzymes such as pectinases, pectin lyases, and proteases that cause macera tion of plant tissue. Some soft-rot Erwinia spp, also harbor genes homologo us to the hypersensitive reaction and pathogenesis (hrp) gene cluster, enco ding components of the type III secretion system. The hrp genes are essenti al virulence determinants for numerous nonmacerating gramnegative plant pat hogens but their role in the virulence of soft-rot Erwinia spp, is not clea r. We isolated and characterized 11 hrp genes of Erwinia carotovora subsp, carotovora. Three putative sigma (1)-dependent Hrp box promoter sequences w ere found. The genes were expressed when the bacteria were grown in Hrp-ind ucing medium. The operon structure of the hrp genes was determined by mRNA hybridization, and the results were in accordance with the location of the Hrp boxes. An E, carotovora strain with mutated hrcC, an essential hrp gene , was constructed. The hrcC(-) strain was able to multiply and cause diseas e in Arabidopsis, but the population kinetics were altered so that growth w as delayed during the early stages of infection.