A Second Cluster of Genes that Specify Pathogenicity and Host Response in Pseudomonas solanacearum. Yong Huang. Department of Plant Pathology, University of Wisconsin, Madison 53706 U.S.A. Peilin Xu, and Luis Sequeira. Department of Plant Pathology, University of Wisconsin, Madison 53706 U.S.A. MPMI 3:48-53. Accepted 17 October 1989. Copyright 1990 The American Phytopathological Society.

Two Tn5-generated mutants of Pseudomonas solanacearum (BOC34B from the nonpathogenic strain B1 and KD688 from the wild-type strain K60) were unable to cause rapid browning or hypersensitive responses (HRs) in callus tissues or leaves, respectively, of the cultivated potato diploid, Solanum phureja. In addition, KD688 lost pathogenicity to potato, eggplant, and tobacco. Genetic analyses showed that Tn5 is inserted in a homologous 7.0-kb EcoRI DNA fragment of both mutants. When the cloned Tn5-containing fragment from BOC34B, pT34, was marker-exchanged into K60, the resulting strains had the same phenotype of the original mutant and showed the same DNA hybridization pattern as BOC34B. With labeled pT34 as a probe, the corresponding wild-type DNA fragment was identified in a K60 genomic library. When a subclone of this fragment was conjugated into the K60 marker exchange mutants, the Path⁺ and HR⁺ phenotypes of the wild-type K60 strain were restored. The DNA region that encodes these functions showed no homology with a previously described hrp cluster in P. solanacearum. Saturation mutagenesis of the 7.0-kb fragment with Tn5-lac revealed two separate transcriptional units of at least 1.3 and 0.7 kb. Insertion of Tn5-lac in either of these units resulted in loss of both pathogenicity and HR functions. When two of these mutants were grown in the presence of plant tissues in either complex or minimal media, expression of the lac gene was induced threefold to sixfold above background levels.