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 alpha-helix indicates that HpaXm is evolutionarily closer to HpaGXag and HpaXac than to Hpa1Xoo and Hpa1Xoc. A synthesized peptide containing two heptads, 39-LDQLLTQ-LIMALLQ-52, from the N-terminal alpha-helical region of HpaXm displayed a comparable activity in inducing HR, but other two synthesized derivatives, HpaXmDeltaT44C and HpaXmDeltaM48Q showed a reduced activity of HR. 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.