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Genes coding for the main virulence determinants of the plant pathogen Erwinia carotovora subsp. carotovora, the plant cell wall-degrading enzymes, are under the coordinate control of global regulator systems including both positive and negative factors. In addition to this global control, some virulence determinants are subject to specific regulation. We have previously shown that mutations in the pehR locus result in reduced virulence and impaired production of one of these enzymes, an endopolygalacturonase (PehA). In contrast, these pehR strains produce essentially wild-type levels of other extracellular enzymes including pectate lyases and cellulases. In this work, we characterized the pehR locus and showed that the DNA sequence is composed of two genes, designated pehR and pehS, present in an operon. Mutations in either pehR or pehS caused a Peh-negative phenotype and resulted in reduced virulence on tobacco seedlings. Complementation experiments indicated that both genes are required for transcriptional activation of the endopolygalacturonase gene, pehA, as well as restoration of virulence. Structural characterization of the pehR-pehS operon demonstrated that the corresponding polypeptides are highly similar to the two-component transcriptional regulators PhoP-PhoQ of both Escherichia coli and Salmonella typhimurium. Functional similarity of PehR-PehS with PhoP-PhoQ of E. coli and S. typhimurium was demonstrated by genetic complementation.

Plasmid pEST4011 enables Pseudomonas putida Paw85 to degrade 2,4-dichlorophenoxyacetic acid (2,4-D) and 3-chlorobenzoate (3-CBA). This new 2,4-D degradative plasmid has considerable homology with the regions of pJP4 containing the 2,4-D degradative genes (tfd). Restriction fragment BamHI-B of plasmid pEST4011, which has homology with this region, was cloned into the broad-host-range vector pKT240 and studied in P. putida PaW85. Restriction mapping, hybridization analysis and enzyme assays established the location of the genes for 2,4-D monooxygenase (tfdA), 2,4-dichlorophenol hydroxylase (tfdB), chlorocatechol 1,2-dioxygenase (tfdC) and the tfdR and tfdS regulatory genes on this fragment. Plasmid pEST4012 is a derivative of pEST4011 derived through the spontaneous deletion of a 42 kbp DNA fragment, which results in the loss of the 2,4-D+ and 3-CBA+ phenotype. We present here the physical maps of pEST4011 and pEST4012. In spite of the similarities in functions, the size (70 kbp), order of catabolic genes and restriction pattern of pEST4011 are clearly different from those of pJP4.

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Erwinia carotovora subsp. carotovora, a Gram-negative phytopathogenic bacterium, secretes an extracellular metalloprotease, PrtW. Previous results demonstrated that protease activity is necessary for the normal progression of disease symptoms caused by this bacterium. The present study revealed that the prtW gene constitutes an independent transcriptional unit. It is demonstrated that introduction of the prtW M plasmid in trans into the prtW N mutant restores the protease activity in this strain. Gene fusions to the gusA (β-glucuronidase) reporter were employed to analyse the transcription of prtW. The transcription of prtW is dependent on many environmental signals. When the bacteria were grown in the presence of potato extract, the expression of the protease gene was markedly higher at the beginning of the exponential phase of growth than that observed when cells were grown in the presence of polygalacturonate (PGA). Analysis of the promoter revealed that an essential regulatory region resided between 371 and 245 bp 5' of the translational start site. As this sequence showed no homology to the KdgR box it may be involved in the binding of an unknown negative regulator protein in E. carotovora subsp. carotovora. The differential responses of prtW expression to potato extract and to PGA appeared to be dependent on the KdgR repressor and the response regulator ExpA. According to the results presented here, it is conceivable that the multiple regulatory network allows flexibility in the expression of the prtW gene during different stages of infection.