Organization and environmental regulation of the Pseudomonas syringae pv. syringae 61 hrp cluster.

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The ability of Pseudomonas syringae pv. syringae 61 to elicit the hypersensitive response in nonhost plant species has been linked to a cluster of hrp/hrm genes whose expression appears to be environmentally regulated. To understand the genetic organization of this hrp/hrm gene cluster and its expression during the interaction with nonhost plant species better, we constructed a set of chromosomal hrp-uidA fusions in P. syringae pv. syringae 61 by Tn5-gusA1 mutagenesis of the cloned hrp/hrm gene cluster and transferred them into the genome by marker exchange mutagenesis. Complementation analysis employing plasmid-borne Tn5-gusA1 insertions and previously characterized chromosomal TnphoA mutations defined at least eight apparent transcriptional units within the hrp/hrm cluster, several of which were multicistronic. The expression of hrp-uidA fusions in seven of these apparent hrp transcriptional units increased following inoculation into tobacco leaves. Enhanced expression from a representative fusion was detected 1 h after inoculation of tobacco leaves. The induction observed in planta was similar to the levels detected following culture of the bacteria in minimal-salts medium: irrespective of the carbon source. Complex amino acid sources, such as peptone, repressed the expression of P. syringae pv. syringae 61 hrp genes at levels exceeding 0.028%. The results indicate that enhanced expression of hrp genes occurs early in the interaction with nonhost plant species in an apparent response to altered nutritional conditions.

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