IDENTIFICATION OF A PUTATIVE ALTERNATE SIGMA-FACTOR AND CHARACTERIZATION OF A MULTICOMPONENT REGULATORY CASCADE CONTROLLING THE EXPRESSION OF PSEUDOMONAS-SYRINGAE PV SYRINGAE PSS61 HRP AND HRMA GENES

The Pseudomonas syringae hrp and hrmA genes controlling pathogenicity and elicitation of the hypersensitive response and the avr genes contr olling host range have been shown previously to be regulated by carbon , nitrogen, pH, osmolarity, and hypothetical plant factors. In P. syri ngae pv. syringae Pss61, inactivation of hrp complementation groups II and XIII reduced expression of a plasmid-borne hrmA'-lacZ fusion. The hrp regions II and XIII were cloned on separate plasmids and shown to enhance the activity of the hrmA promoter in Escherichia coli MC4100 transformants at least 100-fold. The nucleotide sequence of region XII I revealed two open reading frames (hrpR and hrpS) whose deduced produ cts share homolog,vith P. syringae pv. phaseolicola NPS3121 HrpS and a re both related to the NtrC family of two-component signal transductio n systems. HrpR and HrpS differ from most members of the protein famil y by lacking an amino-terminal domain which modulates the regulatory a ctivity. A single open reading frame, hrpL, whose product shares homol ogy with AlgU, a putative alternate sigma factor of P. aeruginosa, as well as with the related alternate sigma factors was identified within region II. Key domains are partially conserved. Inactivation of hrpS in Pss61 repressed expression of a plasmid-borne hrpL'-lacZ fusion car ried by pYXPL1R, and transformation of MC4100(pYXPL1R) with a plasmid carrying hrpRS increased hrpL promoter activity at least 200-fold. Nei ther hrpS nor hrpR, when cloned on separate plasmids, activated the hr pL promoter activity individually. The expression of hrpL when directe d by a Inc promoter was sufficient to express a set of plasmid-borne h rmA'-, hrpJ'-, and hrpZ'-lacZ fusions independently of other hip genes . The results indicate that hrpRS and hrpL are part of a regulatory ca scade in which HrpR and HrpS activate expression of hrpL and HrpL, a p utative sigma factor, induces expression of HrpL-responsive genes.