Open Access Highly Accessed Research article

Phylogenetic analysis of a gene cluster encoding an additional, rhizobial-like type III secretion system that is narrowly distributed among Pseudomonas syringae strains

Anastasia D Gazi123, Panagiotis F Sarris14, Vasiliki E Fadouloglou15, Spyridoula N Charova1, Nikolaos Mathioudakis1, Nicholas J Panopoulos1 and Michael Kokkinidis12*

Author Affiliations

1 Department of Biology, University of Crete, Vasilika Vouton, P.O. Box 2208, Heraklion, Crete GR 71409, Greece

2 Institute of Molecular Biology & Biotechnology, Foundation of Research & Technology, Vasilika Vouton, Heraklion, Crete GR 71110, Greece

3 Institute Pasteur, 28 rue du Docteur Roux, Paris, 75015, France

4 School of Medicine, University of Crete, Vasilika Vouton, Heraklion, Crete, GR 71409, Greece

5 Department of Molecular Biology and Genetics, Democritus University of Thrace, Alexandroupolis, GR 68100, Greece

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BMC Microbiology 2012, 12:188  doi:10.1186/1471-2180-12-188

Published: 2 September 2012



The central role of Type III secretion systems (T3SS) in bacteria-plant interactions is well established, yet unexpected findings are being uncovered through bacterial genome sequencing. Some Pseudomonas syringae strains possess an uncharacterized cluster of genes encoding putative components of a second T3SS (T3SS-2) in addition to the well characterized Hrc1 T3SS which is associated with disease lesions in host plants and with the triggering of hypersensitive response in non-host plants. The aim of this study is to perform an in silico analysis of T3SS-2, and to compare it with other known T3SSs.


Based on phylogenetic analysis and gene organization comparisons, the T3SS-2 cluster of the P. syringae pv. phaseolicola strain is grouped with a second T3SS found in the pNGR234b plasmid of Rhizobium sp. These additional T3SS gene clusters define a subgroup within the Rhizobium T3SS family. Although, T3SS-2 is not distributed as widely as the Hrc1 T3SS in P. syringae strains, it was found to be constitutively expressed in P. syringae pv phaseolicola through RT-PCR experiments.


The relatedness of the P. syringae T3SS-2 to a second T3SS from the pNGR234b plasmid of Rhizobium sp., member of subgroup II of the rhizobial T3SS family, indicates common ancestry and/or possible horizontal transfer events between these species. Functional analysis and genome sequencing of more rhizobia and P. syringae pathovars may shed light into why these bacteria maintain a second T3SS gene cluster in their genome.

Hrc1Type III secretion system; Pseudomonas syringae; Rhizobium Type III secretion system; Phylogenetic analysis; Pathogenicity; Gene organization; Horizontal transfer events; Common ancestry; Evolutionary relationships; RT-PCR