THE PIR GENE OF ERWINIA-CHRYSANTHEMI EC16 REGULATES HYPERINDUCTION OFPECTATE LYASE VIRULENCE GENES IN RESPONSE TO PLANT SIGNALS

The plant pathogenic bacterium Erwinia chrysanthemi secretes pectate l yase proteins that are important virulence factors attacking the cell walls of plant hosts. Bacterial production of these enzymes is induced by the substrate polypectate-Na (NaPP) and further stimulated by the presence of plant extracts. The bacterial regulator responsible for in duction by plant extracts was identified and purified by using a DNA-b inding assay with the promoter region of pelf that encodes a major pec tate lyase. A novel bacterial protein, called Pir, was isolated that p roduced a specific gel shift of the pelf promoter DNA, and the corresp onding pir gene was cloned and sequenced. The Pir protein contains 272 amino acids with a molecular mass of 30 kDa and appears to function a s a dimer. A homology search indicates that Pir belongs to the IclR fa mily of transcriptional regulators. Pir bound to a 35-bp DNA sequence in the promoter region of pelE. This site overlaps that of a previousl y described negative regulator, KdgR Gel shift experiments showed that the binding of either Pir or KdgR interfered with binding of the othe r protein.