Xanthomonas oryzae Pv. oryzicola Response Regulator VemR Is Co-opted by the Sensor Kinase CheA for Phosphorylation of Multiple Pathogenicity-Related Targets

Two-component systems (TCSs) (cognate sensor histidine kinase/response regulator pair, HK/RR) play a crucial role in bacterial adaptation, survival, and productive colonization. An atypical orphan single-domain RR VemR was characterized by the non-vascular pathogen Xanthomonas oryzae pv. oryzicola (Xoc) is known to cause bacterial leaf streak (BLS) disease in rice. Xoc growth and pathogenicity in rice, motility, biosynthesis of extracellular polysaccharide (EPS), and the ability to trigger HR in non-host tobacco were severely compromised in the deletion mutant strain RΔvemR as compared to the wild-type strain RS105. Site-directed mutagenesis and phosphotransfer experiments revealed that the conserved aspartate (D(56)) residue within the stand-alone phosphoacceptor receiver (REC) domain is essential for phosphorelay and the regulatory activity of Xoc VemR. Yeast two-hybrid (Y2H) and co-immunoprecipitation (co-IP) data identified CheA as the HK co-opting the RR VemR for phosphorylation. Affinity proteomics identified several downstream VemR-interacting proteins, such as 2-oxoglutarate dehydrogenase (OGDH), DNA-binding RR SirA, flagellar basal body P-ring formation protein FlgA, Type 4a pilus retraction ATPase PilT, stress-inducible sensor HK BaeS, septum site-determining protein MinD, cytoskeletal protein CcmA, and Type III and VI secretion system proteins HrpG and Hcp, respectively. Y2H and deletion mutant analyses corroborated that VemR interacted with OGDH, SirA, FlgA, and HrpG; thus, implicating multi-layered control of diverse cellular processes including carbon metabolism, motility, and pathogenicity in the rice. Physical interaction between VemR and HrpG suggested cross-talk interaction between CheA/VemR- and HpaS/HrpG-mediated signal transduction events orchestrating the hrp gene expression.