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Environ Microbiol. 2015 Nov;17(11):4164-76. doi: 10.1111/1462-2920.12684. Epub 2014 Dec 17.

XbmR, a new transcription factor involved in the regulation of chemotaxis, biofilm formation and virulence in Xanthomonas citri subsp. citri.

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Instituto de Ciencia y Tecnología Dr. Cesar Milstein, Fundación Pablo Cassará, CONICET, Saladillo 2468, C1440FFX, Ciudad de Buenos Aires, Argentina.
Instituto de Biología Molecular y Celular de Rosario (IBR)-Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICET). Área Virología, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Ocampo y Esmeralda, S2000FHN, Rosario, Argentina.
Estación Experimental Agroindustrial Obispo Colombres, Av. William Cross, 3150, Las Talitas, Tucumán, Argentina.
School of Microbiology, University College Cork, Cork, Ireland.


Xanthomonas citri subsp. citri (Xcc) is the causal agent of citrus canker. Biofilm formation on citrus leaves plays an important role in epiphytic survival of Xcc. Biofilm formation is affected by transposon insertion in XAC3733, which encodes a transcriptional activator of the NtrC family, not linked to a gene encoding a sensor protein, thus could be considered as an 'orphan' regulator whose function is poorly understood in Xanthomonas spp. Here we show that mutation of XAC3733 (named xbmR) resulted in impaired structural development of the Xcc biofilm, loss of chemotaxis and reduced virulence in grapefruit plants. All defective phenotypes were restored to wild-type levels by the introduction of PA2567 from Pseudomonas aeruginosa, which encodes a phosphodiesterase active in the degradation of cyclic diguanosine monophosphate (c-di-GMP). A knockout of xbmR led to a substantial downregulation of fliA that encodes a σ(28) transcription factor, as well as fliC and XAC0350 which are potential member of the σ(28) regulon. XAC0350 encodes an HD-GYP domain c-di-GMP phosphodiesterase. These findings suggest that XbmR is a key regulator of flagellar-dependent motility and chemotaxis exerting its action through a regulatory pathway that involves FliA and c-di-GMP.

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