Cosmid clone pPT10E9 from Pseudomonas syringae pv. tomato caused P. s pv. glycinea to elicit the HR on leaves of all tested soybean cultivars. The avirulence function of pPT10E9, called avrE, occurred on an 11.3-kb DNA fragment located immediately adjacent to the P. s. pv. tomato hrp gene cluster. Tn3-gus saturation mutagenesis of the avrE locus and adjacent DNA revealed at least four transcriptional units occurring immediately adjacent to the hrpRS locus that were all regulated in a manner similar to hrp genes (induced only in minimal induction media or in planta and required the hrpL and hrpRS loci for expression). Transcriptional units III and IV, but not II or V, were required for avrE function. P. s. pv. tomato DC3000 carrying mutations in each of the four transcripts retained full virulence on tomato leaves and elicited the HR on tobacco and soybean plants. This was unlike strain PT23, where mutation of avrE greatly decreased virulence on tomato leaves. The promoter regions for three of the investigated transcriptional units contained a consensus sequence occurring in the promoter regions of several other P. syringae avirulence and hrp genes. The promoter region of transcriptional unit IV, required for avrE function, did not contain such a sequence, but included an element which may function as a sigma-54 promoter. Introduction of the cloned P. s. pv. tomato avrE locus into five other P. syringae pathovars did not cause them to elicit the HR on their normal host plants.