Identification and expression of the Pseudomonas syringae pv. aptata hrpZPsa gene which encodes an harpin elicitor

A sequence homologous to an internal fragment 0.75 kb BstXI of the Pseudomonas syringae pv. syringae hrpZ gene was identified in Pseudomonas syringae pv. aptata NCPPB 2664, the causal agent of bacterial blight in sugar beet, lettuce and other plants, and in E. coli DH10B (pCCP1069) containing the P. syringae pv. aptata hrp gene cluster. PCR with oligonucleotides, based on the hrpZPss gene and used as primers with the total genomic DNA of P. syringae pv. aptata, amplified a 1 kb fragment that hybridized with the probe in highly stringent conditions. The amplicon was cloned into the pGEM® plasmid vector, amplified in E. coli DH5α and sequenced. The sequence showed 95%, 83% and 61% identity with those of hrpZPss, hrpZPsg and hrpZPst genes encoding the harpins of the P. syringae pv. syringae, glycinea and tomato, respectively. The amplicon was cloned into the pMAL® expression system. The expressed protein, fused with maltose-binding protein, was cleaved with a specific protease factor Xa, and purified using affinity chromatography. On the basis of the amino acid sequence and its ability to induce HR in tobacco leaves, it was identified as a P. syringae pv. aptata harpin.