Genome sequence of the plant-pathogenic bacterium Dickeya dadantii 3937.

Dickeya dadantii is a plant-pathogenic enterobacterium responsible for the soft rot disease of many plants of economic importance. We present here the sequence of strain 3937, a strain widely used as a model system for research on the molecular biology and pathogenicity of this group of bacteria.

Population behavior analysis of dspE and pelD regulation in Erwinia chrysanthemi 3937.

Erwinia chrysanthemi 3937 (Ech3937) is a phytopathogenic bacterium with a wide host range. The pectinolytic enzymes secreted by the bacterium and the type III secretion system (T3SS) are essential for full virulence. We used the green fluorescent protein gene as a reporter to investigate the expression of dspE (a putative T3SS effector) and pelD (a major pectin-degrading enzyme) in populations of Ech3937 under different conditions. Gene expression was analyzed by measuring the fluorescence intensity of individual cells with a fluorescence-activated cell sorter. Ech3937 dspE was induced in minimal medium (MM) with only a portion of Ech3937 cells (43.03%) expressing dspE after 12 h of culture. The nutrient-rich King's medium B did not fully eliminate the expression of dspE; a small percentage of Ech3937 cells (5.55%) was able to express dspE after 12 h of culture in this medium. In all, 68.95% of Ech3937 cells expressed pelD after 12 h of culture in MM supplemented with polygalacturonic acid (PGA). However, 96.34% of Echl31 cells (an hrpL deletion mutant of Ech3937) expressed pelD after 12 h of culture in MM supplemented with PGA. In potato tubers, 6.32% of the bacterial cells expressed dspE 2 h after inoculation, whereas only 0.25% of the cells expressed pelD. However, after 24 h, the percentage of cells expressing pelD (68.48%) was approximately 3.5 times that of cells expressing dspE (19.39%). In contrast to potato tubers, similar proportion of Ech3937 cells expressing dspE (39.34%) and pelD (40.30%) were observed in Chinese cabbage 24 h after inoculation. From promoter activity and real-time quantitative results, the expression of pelD in Ech3937 was demonstrated to be downregulated by HrpL in MM supplemented with PGA.

Genome-wide identification of plant-upregulated genes of Erwinia chrysanthemi 3937 using a GFP-based IVET leaf array.

A green fluorescent protein-based in vivo expression technology leaf array was used to identify genes in Erwinia chrysanthemi 3937 that were specifically upregulated in plants compared with growth in a laboratory culture medium. Of 10,000 E. chrysanthemi 3937 clones, 61 were confirmed as plant upregulated. On the basis of sequence similarity, these were recognized with probable functions in metabolism (20%), information transfer (15%), regulation (11%), transport (11%), cell processes (11%), and transposases (2%); the function for the remainder (30%) is unknown. Upregulated genes included transcriptional regulators, iron uptake systems, chemotaxis components, transporters, stress response genes, and several already known or new putative virulence factors. Ten independent mutants were constructed by insertions in these plant-upregulated genes and flanking genes. Two different virulence assays, local leaf maceration and systemic invasion in African violet, were used to evaluate these mutants. Among these, mutants of a purM homolog from Escherichia coli (purM::Tn5), and hrpB, hrcJ, and a hrpD homologs from the Erwinia carotovorum hrpA operon (hrpB::Tn5, hrcJ::Tn5, and hrpD::Tn5) exhibited reduced abilities to produce local and systemic maceration of the plant host. Mutants of rhiT from E. chrysanthemi (rhiT::Tn5), and an eutR homolog from Salmonella typhimurium (eutR::TnS) showed decreased ability to cause systemic inva sion on African violet. However, compared with the wild-type E. chrysanthemi 3937, these mutants exhibited no significant differences in local leaf maceration. The pheno type of hrpB::Tn5, hrcC::Tn5, and hrpD::Tn5 mutants further confirmed our previous findings that hrp genes are crucial virulence determinants in E. chrysanthemi 3937.

First crystallographic structure of a xylanase from glycoside hydrolase family 5: implications for catalysis.

The room-temperature structure of xylanase (EC 3.2.1.8) from the bacterial plant pathogen Erwinia chrysanthemi expressed in Escherichia coli, a 45 kDa, 413-amino acid protein belonging to glycoside hydrolase family 5, has been determined by multiple isomorphous replacement and refined to a resolution of 1.42 A. This represents the first structure of a xylanase not belonging to either glycoside hydrolase family 10 or family 11. The enzyme is composed of two domains similar to most family 10 xylanases and the alpha-amylases. The catalytic domain (residues 46-315) has a (beta/alpha)(8)-barrel motif with a binding cleft along the C-terminal side of the beta-barrel. The catalytic residues, Glu165 and Glu253, determined by correspondence to other family 5 and family 10 glycoside hydrolases, lie inside this cleft on the C-terminal ends of beta-strands 4 and 7, respectively, with an O(epsilon)2...O(epsilon)1 distance of 4.22 A. The smaller domain (residues 31-43 and 323-413) has a beta(9)-barrel motif with five of the strands interfacing with alpha-helices 7 and 8 of the catalytic domain. The first 13 N-terminal residues form one beta-strand of this domain. Residues 44, 45, and 316-322 form the linkers between this domain and the catalytic domain.

The P34 syringolide elicitor receptor interacts with a soybean photorespiration enzyme, NADH-dependent hydroxypyruvate reductase.

The syringolide receptor P34 mediates avrD-Rpg4 gene-for-gene complementarity in soybean. However, the mechanism underlying P34 signal transmission after syringolide binding is unknown. In an effort to identify a second messenger for P34, soybean leaf proteins were run though a P34-affinity column. A 42-kDa protein which specifically bound to the column was identified as a putative plant NADH-dependent hydroxypyruvate reductase (HPR) by N-terminal peptide sequencing. HPR is an important enzyme involved in the plant photorespiration system. Screening of a soybean cDNA library yielded two distinct HPR clones that encoded proteins with 97% identity (P42-1 and P42-2). Surprisingly, only P42-2 displayed good binding with P34 in a yeast two-hybrid assay, indicating that P42-2, but not P42-1, is a potential second messenger for P34. Glycerate and its analogs, which are utilized in the photorespiration system, were tested for their inhibitory effect on syringolide-induced hypersensitive response (HR) to evaluate the biological significance of P42-2. Interestingly, the downstream products of HPR (glycerate and 3-phosphoglycerate) inhibited HR but the upstream compounds (hydroxypyruvate or serine) did not have a significant effect on HR. These results suggest that P42-2 is a primary target for a P34/syringolide complex and that P42-2 binding with the complex probably induces HR by inhibiting one or more HPR functions in soybean.

Microarray profiling of Erwinia chrysanthemi 3937 genes that are regulated during plant infection.

Microarray technology was used to identify genes in Erwinia chrysanthemi 3937 that are specifically up- or down-regulated in a plant host compared with growth in laboratory culture medium. Several genes were plant down-regulated, and almost all of them were homologues of well-known housekeeping genes, such as those encoding metabolic functions, oxidative phosphorylation components, and transcription or translation processes. On the other hand, almost all of the plant up-regulated genes were involved with specialized functions, including already known or new putative virulence factors, anaerobiosis, iron uptake, transporters or permeases, xenobiotic resistance, chemotaxis, and stress responses to reactive oxygen species and heat. A substantial number of the plant up-regulated genes do not appear to be directly involved in damaging the host, but are probably important in adapting the pathogen to the host environment. We constructed insertion mutations in several of the plant up-regulated E. chrysanthemi 3937 genes. Among these, mutations of Bacillus subtilis pps1, Escherichia coli purU, and Pseudomonas aeruginosa pheC homologues reduced virulence on African violet leaves. Thus, new insights were obtained into genes important in bacterial virulence.

hrp genes of Erwinia chrysanthemi 3937 are important virulence factors.

We developed improved virulence assays for Erwinia chrysanthemi 3937 on African violet varieties and devised a new method for the construction of precise bacterial gene knockouts. These methods were tested by constructing mutations in genes suspected to be involved with plant interactions. The virulence of the hrpG and hrcC mutant strains (both gene products presumed to be involved in protein secretion) was greatly reduced on leaves of semitolerant African violet varieties. An hrpN mutant strain produced delayed symptoms on African violet leaves and an hrpN delta pel (delta pel = five major pectate lyase genes deleted) double mutant was nonpathogenic. The hrcC and hrpG mutants did not produce a rapid hypersensitive response (HR) in tobacco, unlike the wild-type bacterium, and the hrpN mutant gave a reduced HR. The results, therefore, establish the importance of hrp genes in the virulence of E. chrysanthemi and their ability to elicit HR on nonhosts. The data also suggest that other effector proteins secreted by the Hrp system are required for full virulence and HR elicitation.