The AraC-type transcription factor TagK is a new player in the signaling cascade that induces the anti-eukaryotic T6SS of Xanthomonas citri.

Type 6 secretion systems (T6SSs) are specialized multiprotein complexes that inject protein effectors into prokaryotic and/or eukaryotic cells. We previously described the role of the T6SS of the phytopathogen Xanthomonas citri pv. citri as an anti-eukaryotic nanoweapon that confers resistance to predation by the amoeba Dictyostelium discoideum. Transcription of the X. citri T6SS genes is induced by a signaling cascade involving the Ser/Thr kinase PknS and the extracytoplasmic function sigma factor EcfK. Here, we used a strain overexpressing a phosphomimetic constitutively active version of EcfK (EcfK T51E ) to identify the EcfK regulon, which includes a previously uncharacterized transcription factor of the AraC-family (TagK), in addition to T6SS genes and genes encoding protein homeostasis factors. Functional studies demonstrated that TagK acts downstream of EcfK, binding directly to T6SS gene promoters and inducing T6SS expression in response to contact with amoeba cells. TagK controls a small regulon, consisting of the complete T6SS, its accessory genes and additional genes encoded within the T6SS cluster. We conclude that a singular regulatory circuit consisting of a transmembrane kinase (PknS), an alternative sigma factor (EcfK) and an AraC-type transcriptional regulator (TagK) promotes expression of the X. citri T6SS in response to a protozoan predator.