1Laboratoire de Biologie Moléculaire des Relations Plantes-Microorganismes, INRA-CNRS, BP27, 31326 Castanet-tolosan Cedex, France; 2Groupe de cristallographie biologique, Institut de Pharmacologie et de Biologie Structurale, CNRS, 205 route de Narbonne, 31077 Toulouse Cedex, France
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Accepted 18 June 2002.
A functional analysis of an 11-kb-long region of the genome of the plant-pathogenic bacterium Ralstonia solanacearum, previously identified as an alternative codon usage region (ACUR), reveals that it was probably acquired through horizontal gene transfer. This ACUR encodes an insertion sequence and eight potential proteins, one of which is partially homologous with a host-specificity factor from a plant-pathogenic Erwinia sp., and another, PopP1, which is homologous to members of the YopJ/AvrRxv family of type III-secreted bacterial effectors controlling interaction between bacteria and their hosts. The analysis of mutants affecting all except one of the genes identified in the ACUR showed that only the popP1-deficient strain had an altered phenotype in plant infection tests. This mutant strain became pathogenic to a Petunia line that is resistant to the wild-type strain. Therefore, popP1 behaves as a typical avirulence gene. We demonstrate that PopP1 protein is secreted and that secretion of this protein requires a functional type III-secretion pathway. In contrast to the structural genes for other type III-secreted proteins identified in R. solanacearum, transcription of the popP1 gene is not coregulated with transcription of hrp genes but is constitutive.
type III secretion pathway
The American Phytopathological Society, 2002