Regulation of pathogenicity in Erwinia and Pseudomonas species.

摘要

Species of Erwinia and Pseudomonas are ubiquitous Gram negative bacteria that survive under diverse conditions and cause diseases in a wide range of crops. The production of pathogenicity and virulence factors, is highly regulated by networks of signals and regulators. I isolated an RsmA- mutant of the soft rotting Erwinia carotovora subsp. carotovora (Ecc) that overproduces extracellular pectate lyase (Pel), cellulase, (Cel) polygalacturonase (Peh), protease (Prt), and the mRNAs of pel-1, pel-3 and peh-1 in both salts-yeast extract-glycerol (SYG) medium and SYG + celery extract medium. It is hypervirulent on celery petioles. The cloned wild-type rsmA+ allele in both RsmA+ and RsmA - strains suppresses the production of Pel, Peh, Cel, Prt and N-(3-oxohexanoyl)-L-homoserine lactone (AHL), the diffusible, cell density (quorum) sensing signal. RsmAEcc+ Ahl Ecc- strain produces drastically lower levels of enzymes, demonstrating the requirement of AHL for enzyme production. The RsmA Ecc- AhlEcc- strain produces enzymes and macerates plant tissues like its RsmAEcc - AhlEcc+ counterpart, indicating that AHL is not required in the RsmA- background.;Southern hybridization analysis revealed the presence of putative rsmAEa and rsmBEa in the non-soft rotting, non-AHL-producing E. amylovora. The rsmA Ea clone was isolated and found to be functionally similar to rsmAEcc as it suppressed AHL, extracellular enzymes, motility and pathogenicity in E. c. carotovora. rsmB Ea was isolated through its reversal of rsmAEa action. The sequences of deduced rsmAEa and of rsmBEa have high homology with their homologs in E. carotovora subsp.carotovora and E. coli, i.e., rsmAEcc and CsrA, and rsmBEcc and crsB, respectively. Like the E. carotovora subsp. carotovora and E. coli homologs, rsmBEa is a non-coding regulatory RNA. The gene pair in E. amylovora controls EPS production, motility and pathogenicity.;Fluorescent plant pathogenic pseudomonads were surveyed for production of AHLs and like the E. c. carotovora strains, Pseudomonas syringae pvs. syringae (Pss), tabaci and tomato as well as P. corrugata and P. savastanoi produce AHLs. The ahlPss locus which specifies AHL production was found to encode a deduced product which has high homology with homologs of LuxI, the AHL synthase from Vibrio fisheri. Chromosomal ahII::lacZ mutants of Pss neither produce AHL nor significant levels of beta-galactosidase, but the expression of ahlIPss::lacZ is substantially stimulated in the presence of multiple copies of the ahlIPss+ DNA or by the addition of cell-free spent cultures containing AHL. These strains were not affected in elicitation of HR and the production of extracellular protease and phytotoxin, syringomycin. Pss strains deficient in the global regulatory genes, gacA or gacS (lemA), produce very low levels of AHL, implying that these regulators control AHL production. Southern analysis revealed that many plant pathogenic pseudomonads, including some that did not produce detectable AHL, possess genetic homologs of ahlI Pss+.