Organization and Expression of the hrp Gene Cluster in Pseudomonas Syryngae PV. Phaseolicola

Abstract

Lindgren et al. (1986) described a group of genes designated hrp, which control the ability of the halo blight pathogen, Pseudomonas syringae pv. phaseolicola, to elicit the hypersensitive reaction (HR) on tobacco and other non-host plants and its ability to produce the primary disease symptoms (watersoaked lesions) on bean, its normal host. Subsequent studies (Lindgren, to be published; Grimm and Panopoulos, unpublished data) showed that hrp DNA sequence are conserved among several pathovars of P. syringae and that functional homologues of several hrp genes exist in closely related members of this group. In P. syringae pv. phaseolicola NPS3121 the hrp genes are organized in 2 linkage groups. One of these apparently comprises a single locus, mutations in which lead to a mucoid morphology. The other group, which comprises the majority of known hrp genes, defines a region of approximately 22 kb which is referred to as the “hrp cluster”. This region has been analyzed further by insertional mutagenesis with “reporter” transposons, marker exchange, complementation and DNA sequence analysis. The hpr cluster consists of two sub-clusters of hrp genes separated by a short (ca. 0.1–1.0 kb) segment that does not control pathogenicity or HR.