The Pseudomonas syringae Hrp pathogenicity island has a tripartite mosaic structure composed of a cluster of type III secretion genes bounded by exchangeable effector and conserved effector loci that contribute to parasitic fitness and pathogenicity in plants

The plant pathogenic bacterium Pseudomonas syringae is divided into pathova rs differing in host specificity, with P. syringae pv. syringae (Psy) and P . syringae pv. tomato (Pto) representing particularly divergent pathovars, P. syringae hrp/hrc genes encode a type III protein secretion system that a ppears to translocate Avr and Hop effector proteins into plant cells. DNA s equence analysis of the hrp/hrc regions in Psy 61, Psy B728a, and Pto DC300 0 has revealed a Hrp pathogenicity island (Pai) with a tripartite mosaic st ructure. The hrp/hrc gene cluster is conserved in all three strains and is flanked by a unique exchangeable effector locus (EEL) and a conserved effec tor locus (CEL). The EELs begin 3 nt downstream of the stop codon of hrpK a nd end, after 2.5-7.3 kb of dissimilar intervening DNA with tRNA(Leu)-queA- tgt sequences that are also found in Pseudomonas aeruginosa but without lin kage to any Hrp Pai sequences, The EELs encode diverse putative effecters, including HopPsyA (HrmA) in Psy 61 and proteins similar to AvrPphE and the AvrB/AvrC/AvrPphC and AvrBsT/AvrRxv/YopJ protein families in Psy B728a. The EELs also contain mobile genetic element sequences and have a G + C conten t significantly lower than the rest of the Hrp Pai or the P. syringae genom e. The CEL carries at least seven ORFs that are conserved between Psy B728a and Pto DC3000, Deletion of the Pto DC3000 EEL slightly reduces bacterial growth in tomato, whereas deletion of a large portion of the CEL strongly r educes growth and abolishes pathogenicity in tomato.