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We identified a compound in culture supernatants of Erwinia species, such as Erwinia amylovora, E. pyrifoliae, E. billingiae, E. tasmaniensis, E. persicina and E. rhapontici absorbing at 340 nm, which was associated before with the yellow pigment produced by E. amylovora on media containing copper ions. The compound was purified from E. tasmaniensis strain Et1/99 supernatants by chromatography on Dowex-1 and Dowex-50 columns and identified by HPLC/MS and NMR analysis as 6-thioguanine (6TG). Its signal at 167 Da matched with the expected molecular mass. By random mutagenesis with miniTn5, we obtained mutants defective in the genes for pyrimidine and purine metabolism. A specific gene cluster with ycf genes described by us before, absent in the corresponding region of Escherichia coli, was identified in the genome sequence of three Erwinia species and named tgs region for thioguanine synthesis. Clones of the tgs gene cluster promoted 6TG synthesis and secretion in E. coli, when the bacteria were grown in minimal medium supplemented with amino acids. 6TG was bacteriostatic for E. coli and Salmonella typhimurium strains, with cell growth resumed after prolonged incubation. Similar results were obtained with P. agglomerans strains. Bacteria from the genus Pectobacterium were barely and Rahnella or Gibbsiella species were not inhibited by 6TG. Adenine and guanine relieved the toxic effect of 6TG on E. coli. Non-producing strains were fully virulent on host plants. 6TG synthesis may help erwinias to interfere with growth of some microorganisms in the environment.

Erwinia amylovora is the causative agent of fire blight, a destructive disease of rosaceous plants. The European population can be divided into several subtypes according to differences in restriction fragment length polymorphism of the XbaI genomic DNA digest analysed with pulsed-field gel electrophoresis. This technique was also used to determine the genetic relatedness of six Croatian isolates to the E. amylovora types found in the countries surrounding Croatia. The isolates belong to the Pt2 pattern type that is characteristic of the East Mediterranean basin. All tested isolates gave essentially the same total cell protein pattern in SDS-polyacrylamide gel electrophoresis. The number of short-sequence DNA repeats in plasmid pEA29 of six isolates was determined by PCR assays and ranged from four to seven. The isolates examined showed high pathogenicity in immature pear fruits. Differences were also revealed in microbiological assays such as amylovoran synthesis, levan formation, siderophore production and colour on coliform medium.

Transgenic plants, expressing recombinant proteins, are suitable alternatives for the production of relevant immunogens. In the present study, the expression of Puumala virus nucleocapsid protein in tobacco and potato plants (Nicotiana tabacum and Solanum tuberosum) and its immunogenicity was investigated. After infection of leaf discs of SR1 tobacco and tuber discs of potato cv. “Desiree” with the Agrobacterium strain LBA4404 (pAL4404, pBinAR-PUU-S) containing the 1302 bp cDNA sequence of S-RNA segment of a Puumala virus, transgenic tobacco and potato plants expressed the Puumala virus nucleocapsid protein under control of the cauliflower 35S promoter. The recombinant proteins were found to be identical to the authentic Puumala virus nucleocapsid protein as analyzed by immunoblotting. Expression of the nucleocapsid protein was investigated over four plant generations (P to F4) and found to be stable (1 ng/3 μg dried leaf tissue). Transgenic tobacco plants were smaller compared to controls. The transformed potato plants were morphologically similar to control plants and produced tubers as the control potatoes. The S-antigen was expressed at a level of 1 ng protein/5 μg and 1 ng protein/4 μg dried leaf and root tissues, respectively, and remained stable in the first generation of vegetatively propagated potato plants. The immunogenicity of the Puumala virus nucleocapsid protein expressed in Nicotiana tabacum and Solanum tuberosum was investigated in New Zealand white rabbits. They were immunized with leaf extracts from transgenic tobacco and potato plants, and the serum recognized Puumala virus nucleocapsid protein. Transgenic plants expressing hantaviral proteins can thus be used for the development of cost-effective diagnostic systems and for alternative vaccination strategies.

We quantified the expression of the puf and puc operons, which encode pigment binding proteins, in a number of strains of Rhodobacter capsulatus that have defects in genes affecting different steps of bacteriochlorophyll biosynthesis. Our results show that these mutations have a very similar effect on puf and puc expression. This suggests that the reduced expression of genes encoding pigment-binding proteins is due neither to the accumulation of a specific intermediate of bacteriochlorophyll synthesis nor to the lack of an early intermediate, but is rather the result of the lack of a very late intermediate or the bacteriochlorophyll molecule itself.

Several Erwinia amylovora strains isolated in the Netherlands from apple and pear trees and other hosts for fireblight were found to be conditionally deficient in levan synthesis. The levansucrase genes of five strains were cloned via PCR amplification, and the encoded enzyme was expressed in Escherichia coli. Levan was also produced from the cloned genes in high copy number plasmids, when they were transformed into the corresponding levan-deficient strains. The expression was not driven by the promoter in the cloning vector. An lsc transcript was detected by northern blot analysis with an intermediate signal for strain PD207 and, surprisingly, with a strong signal for PD576. The other four strains did not produce a hybridization signal. The nucleotide sequence in front of the promoter region, when assayed for all five deficient strains, was identical with the sequence previously determined from wild type strain Ea7/74. Also strain E8, which is deficient in levan and in amylovoran synthesis, carries an intact lsc -gene. At 28 °C the expression of lsc from the deficient strains was low in several media with various nutrients and sugars. However, when the strains were grown at 18 °C, levan synthesis was partially restored, indicating a temperature-dependent regulation of lsc -expression.

Summary In order to determine a possible genomic divergence of Erwinia amylovora‘fruit tree’ and raspberry strains from North America, several isolates were differentiated by pulsed-field gel electrophoresis (PFGE) analysis, the size of short DNA sequence repeats (SSRs) and the nucleotide and deduced amino acid sequences of their hrpN genes. By PFGE analysis European strains are highly related, whereas strains from North America were diverse and were further distinguished by the SSR numbers from plasmid pEA29. The E. amylovora strains from Europe showed identical HrpN sequences in contrast to the American isolates from fruit trees and raspberry. Those were related to each other, but distinguishable by their HrpN patterns. The Asian pear pathogens differed in HrpN among each other and from E. amylovora. Erwinia pyrifoliae isolates and the Erwinia strains from Japan were ordered via their HrpN sequences in agreement with the PFGE patterns. For all three pathogens, dendrograms from PFGE and sequence data indicate an evolutionary diversity within the species in spite of a genetic conservation for parts of the hrpN genes suggesting a long persistence of the Asian pear pathogens in Korea and Japan as well as of fire blight in North America. Some of the divergent American E. amylovora isolates share PFGE patterns with the relatively uniform European strains.

An array of short-sequence DNA repeats (SSRs) occurs in the plasmid pEA29 of the fire blight pathogen Erwinia amylovora. A large number of "fruit tree" strains, mainly from Central and Western Europe, were screened for their SSR numbers, and the analyses were extended to five raspberry strains from North America and six pear pathogenic Erwinia strains from Japan. The repeat ATTACAGA present in all E. amylovora strains was found to be reiterated 3 to 15 times. The Japanese strains contained the major repeat sequence GGATTCTG, which was reiterated 16 to 24 times. ATTACAGG, which resembles the SSR of E. amylovora, was reiterated two or three times. In a novel approach, sequencing gels were used to visualize the rare occurrence of shorter arrays (down to three repeats) in E. amylovora and the Japanese Erwinia strains. Changes in the repeat numbers in E. amylovora were observed repeatedly when the bacteria had been exposed to stress conditions. The repeat structures of homo- and heteroduplices of PCR-amplified repeats were also analyzed by cleavage of annealed molecules with the single-strand-specific endonuclease from bacteriophage T4. Not only heteroduplexes, but also homoduplexes showed non-matching regions in the SSRs, which could arise from transient formation of loops due to strand slippage during the assays.

In order to find reasons for the absence of fire blight in most countries of the Southern hemisphere, bark samples from apple and pear trees in orchards of the Western Cape region in South Africa were extracted for bacteria which could be antagonistic to Erwinia amylovora. Screening was done in the late growth season and mainly Gram-positive bacteria were isolated. Approximately half of them produced growth inhibition zones on a lawn of E. amylovora. Most isolates were classified as Bacillus megaterium by microbiological assays and in API 50 test systems. They were visualized in the light microscope as non-motile large rods. These strains may not be responsible for the absence of fire blight in orchards, but they may indicate unfavourable climatic conditions for Gram-negative bacteria including E. amylovora. They may reduce the ability of E. amylovora to establish fire blight and could also be useful for application in biological disease control.

Most Erwinia amylovora strains form yellow mucoid colonies on solid minimal medium containing asparagine and copper sulfate (MM2Cu). One exception is the strain Ea25/82, which produces white colonies on MM2Cu agar. This strain was transformed with a genomic library of E. amylovora and yellow colonies were recovered. A 1.5-kb fragment was found to complement strain Ea25/82 for color formation, and subsequent sequencing revealed two ORFs. The smaller ORF132(ycfB) overlapped with the end of the larger ORF253(ycfA). The putative protein YcfA shows low homology with K+/Na+ channel transporter ATPases. Resistance genes were inserted in both ORFs, and the E. amylovora strains Ea1/79-YA and Ea1/79-YB were created by site-directed mutagenesis. The mutation in ycfB did not affect color formation, whereas the ycfA mutant formed white colonies on MM2Cu. Sequence analysis of the ycf region in strain Ea25/82 revealed a 1-bp alteration in ycfA and no change in ycfB. Stable complementation of Ea25/82 and Ea1/79-YA, however, required both genes. Carotenoids were not detected in E. amylovora grown in the presence of copper ions. On the other hand, copper-independent secretion of a low-molecular-weight compound with an absorption maximum at 340 nm (CP340) was found for strain Ea1/79, but not for Ea25/82 or the mutant Ea1/79-YA. CP340 formed a complex with copper ions, and complementation with plasmids carrying both ycfA and ycfB restored its release from mutant strains. The compound may be connected with the yellow pigment or function in sensing bacterial population densities.

In Rhodobacter capsulatus the puf operon encodes proteins of the photosynthetic apparatus. The polycistronic puf mRNA is comprised of segments that show differential stability. Here, we show that the rate of decay of the 2.7-kb pufBALMX mRNA species in Escherichia coli depends on the activity of ribonuclease E (RNase E), whereas the degradation of the 0.5-kb pufBA mRNA segment is not affected by a mutation in the rne gene. The RNase E-promoted decay of the pufLMX mRNA depends on the presence of a 1.4-kb pufLM mRNA segment, in which rate-limiting endonucleolytic cleavage was postulated to occur in R. capsulatus. The insertion of 185 bp of this 1.4-kb segment into pufB results in an RNase E-dependent decay of the modified pufBA mRNA segment in E. coli. Our findings suggest that in R. capsulatus an RNase E-like activity is responsible for the rate-limiting endonucleolytic cleavage occurring within the pufLM mRNA segment, whereas the 0.5-kb pufBA mRNA segment is degraded by a different RNase E-independent decay mechanism.

A DNA sequence with dyad symmetry upstream of the transcriptional start of the Rhodobacter capsulatus puf operon, which encodes pigment-binding proteins of the light-harvesting I complex and of the reaction center, has previously been shown to be a protein-binding site (G. Klug, Mol. Gen. Genet. 226:167-176, 1991). When a low-copy-number plasmid with a base pair transition at position -43 within this dyad symmetry in front of the puf structural genes was transferred into a Rhodobacter strain with the puf operon deleted, different phenotypes occurred during cultivation of the transconjugants and the kinetics of the loss of the wild-type phenotype was dependent on the oxygen tension in the culture. After growth for 150 generations, the different phenotypes were stably inherited. The strains having the wild-type phenotype carried the wild-type puf DNA sequence. The original mutation was still present in the strains that showed lighter color. These strains had less light-harvesting II complex in the membrane and showed lower rates of transcription of the puc operon, which encodes the proteins of this complex. This deregulation of puc expression was due to one or more chromosomally located, secondary mutations, not directly to the mutation present on the plasmid. Thus, a single-base-pair transition in the puf upstream region can result in a deregulation of puc expression, suggesting a direct or indirect transcriptional coregulation of both these operons by a common factor.