A proteinaceous elicitor of the plant defense reaction known as the hypersensitive response was isolated from Erwinia amylovora, the bacterium that causes fire blight of pear, apple, and other rosaceous plants. The elicitor, named harpin, is an acidic, heat-stable, cell-envelope-associated protein with an apparent molecular weight of 44 kilodaltons. Harpin caused tobacco leaf lamina to collapse and caused an increase in the pH of bathing solutions of suspension-cultured tobacco cells. The gene encoding harpin (hrpN) was located in the 40-kilobase hrp gene cluster of E. amylovora, sequenced, and mutated with Tn5tac1. The hrpN mutants were not pathogenic to pear, did not elicit the hypersensitive response, and did not produce harpin.

Pantoea agglomerans (synonym: Erwinia herbicola) strain Eh318 produces through antibiosis a complex zone of inhibited growth in an overlay seeded with Erwinia amylovora, the causal agent of fire blight. This zone is caused by two antibiotics, named pantocin A and B. Using a genomic library of Eh318, two cosmids, pCPP702 and pCPP704, were identified that conferred on Escherichia coli the ability to inhibit growth of E. amylovora. The two cosmids conferred different antibiotic activities on E. coli DH5␣ and had distinct restriction enzyme profiles. A smaller, antibiotic-conferring DNA segment from each cosmid was cloned. Each subclone was characterized and mutagenized with transposons to generate clones that were deficient in conferring pantocin A and B production, respectively. Mutated subclones were introduced into Eh318 to create three antibioticdefective marker exchange mutants: strain Eh421 (pantocin A deficient); strain Eh439 (pantocin B deficient), and Eh440 (deficient in both pantocins). Cross-hybridization results, restriction maps, and spectrum-ofactivity data using the subclones and marker exchange mutants, supported the presence of two distinct antibiotics, pantocin A and pantocin B, whose biosynthetic genes were present in pCPP702 and pCPP704, respectively. The structure of pantocin A is unknown, whereas that of pantocin B has been determined as (R)-N-[((S)-2-amino-propanoylamino)-methyl]-2-methanesulfonyl-succinamic acid. The two pantocins mainly affect other enteric bacteria, based on limited testing. Pantoea agglomerans or Pantoea dispersa (20), also known asErwinia herbicola (Löhnis) Dye are members of the Enterobacteriaceae and are ubiquitous in nature, inhabiting plants, soil, and water (16,20,21) and animals and humans (16,35). Strains belonging to E. herbicola are members of the E. herbicolaEnterobacter agglomerans cluster; some have been redesignated P. agglomerans and P. dispersa, while others did not fall into either of the two species (20). P. agglomerans and P. dispersa are frequent companions of Erwinia amylovora (Burr.) Winslow et al. the causal agent of the disease fire blight of apple and pear trees (36, 38). There is current interest in P. agglomerans and P. dispersa as biological control agents for fire blight because they are harmless to apple and pear trees and are able to protect them against invasion of the pathogen (4, 29). P. agglomerans strain Eh318, isolated from a symptomless apple stem in New York State, protected immature pear fruits in the laboratory (53) and apple blossoms in controlled environment and orchard tests (5,23,43).Production of antibiotics inhibitory to E. amylovora by several strains of Pantoea spp. seems important for inhibition of E. amylovora in planta (30,45,53). In vitro inhibition of E. amylovora by antibiotics of Pantoea spp. is well documented (24,28,45,47,48). Different strains of P. agglomerans and P. dispersa have different spectra of antimicrobial activity (15,25) and produce different types of inhibition zones against the same indicator organis...

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The description for three strains of Erwinia cancerogena was expanded to include 108 phenotypic characters. These strains were compared with seven strains of Erwinia carotovora subsp. carotovora and the type strains of Enterobacter (Erwinia) nimipressuralis, Enterobacter cloacae, Enterobacter amnigenus, and Enterobacter intermedium. Our results support the proposal to transfer Erwinia cancerogena to the genus Enterobacter as Enterobacter cancerogenus comb. nov.The bacteria isolated by UroSeviC from poplars (Populus species) affected by a canker disease were described and designated Erwinia cancerogena in 1966 (28). Although Lelliott (18) and Lelliott and Dickey (20) have indicated that Erwinia cancerogena probably is a species of Enterobacter because it produces positive reactions for arginine and ornithine decarboxylase, additional data have not been reported to substantiate the suggested change to Enterobacter.A preliminary comparison of 48 characters showed that the level of similarity between Erwinia cancerogena and Erwinia amylovora is 54%, whereas the level of similarity between Erwinia cancerogena and Erwinia carotovora subsp. carotovora is 85%, which is greater than the levels of similarity for the other 14 species or subspecies of Erwinia (unpublished data). A close relationship between Erwinia cancerogena and Enterobacter (Erwinia) nimipressuralis, Enterobacter cloacae, Enterobacter amnigenus, and Enterobacter intermedium was indicated by a comparison with results reported previously for additional species of the family Enterobacteriaceae (3,6,8,9,15,16,21,26,29).The purpose of this study was to expand the phenotypic characterization of Erwinia cancerogena and to determine its relationship to strains of Erwinia carotovora subsp. carotovora and to the type strains of Enterobacter nimipressuralis, Enterobacter cloacae, Enterobacter amnigenus, and Enterobacter intermedium. Morphological, physiological, and biochemical tests. The strains were stained with the Hucker modification of the Gram staining procedure (lo), and bacterial suspensions were examined by phase-contrast microscopy for cell morphology and motility. Motility test agar (Difco) and motility test medium (9) also were used. The bacteria were negatively stained for 5 to 10 min with a 1:l mixture of 0.5% uranyl acetate and 0.025% bacitracin for examination of flagella by electron microscopy or were stained with Ryu flagellum stain (22) for light microscopy. Pigment production was determined on Difco nutrient agar and yeast extract-dextrose-calcium carbonate agar (7) at 27°C. MATERIALS AND METHODS Bacterial strains. Three strains of Erwinia cancerogena

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