Genome-Wide Identification of a Large Repertoire of Ralstonia solanacearum Type III Effector Proteins by a New Functional Screen

Abstract

The gram-negative plant-pathogenic bacterium Ralstonia solanacearum utilizes the hypersensitive response and pathogenicity (Hrp) type III secretion system (T3SS) to cause disease in plants. To determine the entire repertoire of effector proteins possessed by R. solanacearum RS1000, we constructed a transposon carrying a calmodulin-dependent adenylate cyclase reporter that can be used to specifically detect rip (Ralstonia protein injected into plant cells) genes by monitoring the cAMP level in plant leaves inoculated with insertion mutants. From the new functional screen using this transposon, we identified 38 new Rip proteins translocated into plant cells via the Hrp T3SS. In addition, most of the 34 known effectors of RS1000 could be detected by the screen, except for three effectors that appear to be small in size or only weakly expressed. Finally, we identified 72 Rips in RS1000, which include 68 effector proteins classified into over 50 families and four extracellular components of the Hrp T3SS. Interestingly, one-third of the effectors are specific to R. solanacearum. Many effector proteins contain various repeated amino acid sequences or known enzyme motifs. We also show that most of the R. solanacearum effector proteins, but not Hrp extracellular components, require an Hrp-associated protein, HpaB, for their effective translocation into plant cells.