A novel plant defense gene, hsr203J, whose corresponding mRNA accumulates preferentially during the incompatible interaction of tobacco (Nicotiana tabacum L.) with a pathogenic bacterium, Pseudomonas solanacearum, has been isolated and sequenced. No sequence homology of the putative product of this gene has been found in data bases. Evidence is presented here that the hsr203J gene promoter, when fused to the GUS reporter gene, is selectively expressed in response to the hypersensitive response (HR)-inducing bacteria in tobacco protoplasts and that the sequences responsible for this response are contained within 1.4 kb of the 5' noncoding region. The temporal and spatial patterns of hsr203J activation in leaves and roots inoculated with P. solanacearum indicate that the hsr203J promoter exhibits a rapid (3-6 h post-inoculation) and high level of induction only in plant cells inoculated with the HR-inducing bacterial isolate. In addition, this gene promoter which does not respond to various stress conditions and is only very weakly induced during compatible interactions, is strongly dependent on hrp (hypersensitive response and pathogenicity) genes of P. solanacearum. These data indicate that the hsr203J gene promoter exhibits new and original characteristics of activation with regard to the plant defense genes studied so far; its spatial and temporal program of activation together with its specific induction during the HR underline the importance of this gene as a molecular tool for studying the establishment and regulation of the HR.