Further Characterization of an hrp Gene Cluster of Erwinia amylovora. David W. Bauer. Department of Plant Pathology, Cornell University, Ithaca, NY 14853 U.S.A. Steven V. Beer. Department of Plant Pathology, Cornell University, Ithaca, NY 14853 U.S.A. MPMI 4:493-499. Accepted for publication 18 June 1991. Copyright 1991 The American Phytopathological Society.

Two independent Tn5-induced mutants of Erwinia amylovora, Ea321T102 and Ea322T101, were identified that failed to elicit a hypersensitive response (HR) in a nonhost plant, tobacco. The two also were nonpathogenic on immature pear fruit. Two naturally occurring nonpathogenic strains, P66 and CFPB1376, also were found incapable of eliciting an HR. Three previously reported Tn5-induced nonpathogenic mutants (Steinberger and Beer, Mol. Plant-Microbe Interact. 1:135-144, 1988) were found to elicit a variable HR (Ea321T101 and Ea321T104) or a normal HR (Ea322T104). Two recombinant plasmids and a previously described cosmid containing wild-type E. amylovora DNA restored pathogenicity and the ability to elicit the HR to the seven strains. Restriction mapping and hybridization showed that the cosmid and plasmids overlap; thus, the mutated genes are clustered. Functional analysis of subclones from the two plasmids was used to determine the approximate region of DNA complementing each mutation. These results were combined with some results reported previously and the results of additional tests for complementation. The analysis revealed a cluster of at least six complementation regions involved in pathogenicity of host plants and elicitation of the HR in a nonhost plant.