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Phylogenetic evidence for extensive horizontal gene transfer of type III secretion system genes among enterobacterial plant pathogens

¹ Department of Biological Sciences, University of Illinois at Chicago, Chicago, IL 60611, USA
² Division of Microbiology, Center for Food Safety and Applied Nutrition, Food and Drug Administration (FDA), College Park, MD 20740, USA

This study uses sequences from four genes, which are involved in the formation of the type III secretion apparatus, to determine the role of horizontal gene transfer in the evolution of virulence genes for the enterobacterial plant pathogens. Sequences of Erwinia, Brenneria, Pectobacterium, Dickeya and Pantoea were compared (a) with one another, (b) with sequences of enterobacterial animal pathogens, and (c) with sequences of plant pathogenic and β proteobacteria, to evaluate probable paths of lateral exchange leading to the current distribution of virulence determinants among these micro-organisms. Phylogenies were reconstructed based on hrcC, hrcR, hrcJ and hrcV gene sequences using parsimony and maximum-likelihood algorithms. Virulence gene phylogenies were also compared with several housekeeping gene loci in order to evaluate patterns of lateral versus vertical acquisition. The resulting phylogenies suggest that multiple horizontal gene transfer events have occurred both within and among the enterobacterial plant pathogens and plant pathogenic and β proteobacteria. hrcJ sequences are the most similar, exhibiting anywhere from 2 to 50 % variation at the nucleotide level, with the highest degree of variation present between plant and animal pathogen sequences. hrcV sequences are conserved among plant and animal pathogens at the N terminus. The C-terminal domain is conserved only among the enterobacterial plant pathogens, as are the hrcC and hrcR sequences. Additionally, hrcJ and hrcV sequence phylogenies suggest that at least some type III secretion system virulence genes from enterobacterial plant pathogens are related more closely to those of the genus Pseudomonas, a conclusion neither supported nor refuted by hrcC or hrcR.

Correspondence
Marianna Naum
marianna.naum{at}fda.hhs.gov

The GenBank/EMBL/DDBJ accession numbers for the hrcC, hrcJ, hrcR and hrcV sequences of the strains examined in this study are EU561240–EU561268, EU561269–EU561288, EU561289–EU561316 and EU561317–EU561337, respectively.