Conservation of the Pathogenicity Island for Biosynthesis of the Phytotoxin Phaseolotoxin in Pseudomonas syringae Pathovars

Abstract

Several pathovars of Pseudomonas syringae produce extracellular toxins that often increase their virulence towards plants. Phaseolotoxin is a modi- fied tripeptide that inhibits the key plant enzyme, ornithine carbamoyl transferase (OCTase), and is produced by the P. syringae pvs. phaseolicola and actinidiae as well as by a single strain of P. syringae pv. syringae, CFBP3388. Genes required for the biosynthesis of phaseolotoxin map to a ca. 28-kb genomic region designated as the Pht cluster, which is included in a larger region characteristic for a pathogenicity island. The Pht cluster included argK, a gene coding for an OCTase which confers resistance to phaseolotoxin. Since the sequence of argK is identical among strains of P. syringae pv. phaseolicola and P. syringae pv. actinidiae, others suggested that the argK-tox cluster might have been horizontally acquired. The comparison of the published sequence of four Pht clusters shows a very high level of identity (around 99.8%), although the few occurring nt changes often result in important changes in the corresponding deduced gene products. The sequence and the overall organiza- tion of the Pht cluster and flanking regions is conserved in all examined strains of pathovars phaseolicola and actinidiae that produced phaseolotoxin. However, PCR experiments indicated that the sequence and/or organization of the Pht cluster are only poorly conserved in P. syringae pv. syringae CFBP3388. The sequence of a 2.4-kb fragment from the Pht cluster from this strain, spanning from phtO to amtA, showed an 83% overall identity with the corresponding sequence of P. syringae pv. phaseolicola.1448A. Collectively, our results indicated that the pathogenicity island containing the Pht cluster has evolutionary invaded P. syringae several times.