HpaXm from Xanthomonas citri subsp malvacearum is a Novel Harpin with Two Heptads for Hypersensitive Response

A novel harpin-like protein, HpaXm, was described from cotton leaf blight bacteria, Xanthomonas citri subsp. malvacearum. The hpaXm was found to be localized between hrp2 and hrcC. A phylogenetic analysis of the complete amino acid sequence or solely the 13 highly conserved residues H2N-SEKQLDQLLTQLI-COOH in the N-terminal a-helix indicates that HpaXm is evolutionarily closer to HpaGXag and HpaXac than to Hpa1Xoo and Hpa1Xoc. A synthesized peptide containing two heptads, 39-LDQLLTQLIMALLQ-52, from the N-terminal a-helical region of HpaXm displayed comparable activity in inducing a hypersensitive response, but two other synthesized derivatives, HpaXm Delta T44C and HpaXm Delta M48Q, showed reduced HR-triggering activity. The data from a GST trap test revealed that HpaXm was released into the extracellular medium, hpaXm mutant deficient for the leader peptide (1-MNSLNTQIGANSSFL-15) was unable to be secreted outside cells but still induced HR in tobacco leaves.