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A 25-kb DNA region, previously cloned from Pseudomonas syringae pv. syringae 61 in cosmid pHIR11, enables non-pathogenic bacteria such as Pseudomonas fluorescens and Escherichia coli to elicit the hypersensitive response (HR) in tobacco (Nicotiana tabacum). hrmA is located within this region, adjacent to a conserved cluster of hrp genes, and is essential for nonpathogens to elicit the HR. DNA sequence analysis suggested that hrmA was the second of two genes in an operon and was preceded by an open reading frame (ORF), ORF1, which is predicted to encode a 10.9-kDa protein. DNA gel blot analysis revealed that sequences hybridizing with a DNA fragment internal to hrmA were absent from P. syringae pv. syringae B728a, P. syringae pv. tabaci 11528, and P. syringae pv. glycinea race 4 U1, but present in P. syringae pv. tomato DC3000. A 2.4-kb BamHI-AvrII fragment carrying hrmA, ORF1, and …