ISHS Acta Horticulturae 411: VII International Workshop on Fire Blight
THE HRP GENE CLUSTER OF ERWINIA AMYLOVORA CONTAINS HOMOLOGS OF YSC GENES OF YERSINIA |
| Authors: | A.J. Bogdanove, Z.-M. Wei, L. Zhao, S.V. Beer |
| Abstract:
Erwinia amylovora causes fire blight of apple, pear, and other rosaceous plants. In non-host plants, such as tobacco, E. amylovora elicits the rapid, localized necrosis known as the hypersensitive response (HR). The HR is an active process (He et al., 1994) correlated with plant defense. Genes required by plant pathogenic bacteria both for elicitation of the HR and for pathogenicity are designated "hrp" (Lindgren et al., 1986). Clusters of hrp genes have been cloned from a number of plant pathogens, including E. amylovora, Pseudomonas solanacearum, and several pathovars of Xanthomonas campestris and Pseudomonas syringae. In large part, these were discovered to be physically and functionally conserved (Willis et al., 1991; Laby and Beer, 1992). The hrp gene cluster of E. amylovora was cloned from strain Ea321 in the cosmid pCPP430 and found to confer on E. coli and other non-plant pathogens the ability to elicit an HR in many plants (Beer et al., 1991) and to enable bacterial multiplication and induction of electrolyte leakage and necrosis in apple leaf segments (Zhao et al., unpublished results). Harpin, an extracellular elicitor of the HR and a pathogenicity determinant, was identified as a product of the hrpN gene located within this cluster (Wei et al., 1992a). There are at least eight complementation groups in a 25 kb region of the cluster essential for HR elicitation; one is hrpN, at least two regulate the synthesis of harpin, and the remaining five are necessary for secretion of harpin (Wei and Beer, 1993). We determined the nucleotide sequence of ca. 8 kb region of the E. amylovora hrp gene cluster spanning complementation groups VI, VII, and VII downstream of the previously characterized hrpl gene (Wei and Beer, 1993). The region contains nine contiguous or overlapping open reading frames (ORFs) immediately adjacent to hrpl, followed 64 bp downstream by two small ORFs spaced 58 bp apart.
The translational direction of the ORFs (the same as that of hrpl) is in agreement with that predicted previously using Previous results (Wei and Beer, 1993) indicated homology of Hrpl and members, including LcrD of Yersinia pestis, of a large family of integral membrane proteins involved or implicated in protein secretion. Comparison of the predicted amino acid sequences of the ORFs downstream of hrpl to sequences in the major data bases revealed similarity of many of the ORFs to genes in the virB/IcrB (yscN-yscU) locus of Yersinia spp. involved in secretion of the virulence proteins called Yops (Fields et al., 1994; Bergman et al., 1994). Spatial organization of these ORFs | |
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-glucuronidase reporter gene fusions (Wei et al., 1992b). Each ORF is preceded by an appropriately spaced potential ribosome binding site.
ORF 9 begins with the unusual start codon UUG. No palindromic sequences typical of a rho-independent terminator were found in the sequenced region.
The results suggest that hrpl and ORFs 1–9 are in the same operon, and that the last two ORFs, 10 and 11, may be transcribed independently.
