Redox-responsive LysR-type transcription factors in the phytopathogen Agrobacterium tumefaciens

Agrobacterium tumefaciens, a ubiquitous soilborne phytopathogen, continuously encounters reactive oxygen species (ROS) generated endogenously during respiration or exogenously during host infection. Consequentially, adaptive protection against ROS is inevitable for survival and pathogenicity. A broadly known redox-responsive regulator is the ubiquitously found LysR-type transcription factor OxyR, which directly senses peroxide stress through redox-active cysteine residues. Interestingly, A. tumefaciens harbors a distinct LysR-type transcription factor called LsrB, which putatively plays a role in the oxidative stress response. Unlike OxyR, LsrB is solely found in Rhizobiales and encoded in a polycistronic operon with the thioredoxin reductase gene trxB. We asked if and how LsrB has a similar or even overlapping function with OxyR.

We found that deletion of lsrB and oxyR leads to increased sensitivity to a broad spectrum of reactive oxygen species. Genome-wide transcriptome profiling under hydrogen-peroxide stress revealed that both regulators control a complex network of oxidative stress responsive genes, such as antioxidant systems, as well as numerous small RNAs. Redox state profiling via roGFP biosensors further indicated a more oxidized intracellular redox environment in the lsrB and oxyR mutant, which is in line with the dysregulation of antioxidant systems. In silico structure modeling and functional studies identified redox-active cysteine residues in LsrB, suggesting a similar mode of action as OxyR, highlighting the intricate interplay between the two regulators. In summary, our findings uncover the pivotal role of OxyR and LsrB in orchestrating the adaptive protection against ROS in A. tumefaciens.