Bacterial sensing and response for neutralization and detoxification of environmental ammonia

Ammonia present in the environment is a major source of nitrogen, but it can be toxic to bacteria. While the biochemical mechanisms involved in the metabolic detoxification of cellular ammonia are well understood, little is known about how bacteria manage toxic external ammonia to survive, especially when ammonia is present as a waste product at high concentrations. Here, we demonstrate that a two-component system consisting of the sensor kinase GrtK and the response regulator GrtR is responsible for sensing and neutralizing toxic environmental ammonia produced as a waste product by the rice pathogen Burkholderia glumae. The growth of null mutants of grtK or grtR was inhibited in amino acid-rich media such as Luria-Bertani medium, but no growth inhibition was observed in amino acid-free media. The expression of obcAB, responsible for the biosynthesis of the previously known neutralizing agent oxalate, was dependent upon external ammonia concentration in a GrtR-dependent manner. Significant changes in fluorescence were observed when cells of B. glumae carrying a recombinant plasmid of the modified circular permutation GFP gene fused to grtK were incubated with compounds containing ammonium, suggesting that GrtK interacts selectively with external ammonia. Transcriptome analysis of grtK and grtR mutants also showed that GrtK and GrtR are involved in the metabolic detoxification of cellular ammonia as well. These results indicate that GrtK is an external ammonia sensor that is not a member of the ammonia transporter protein family and works together with the response regulator GrtR to counter the risk posed by its own metabolism.IMPORTANCEExcessive accumulation of external ammonia, resulting from the deamination of amino acids used as carbon sources, can be toxic to bacteria. However, there is a limited understanding of how bacteria sense toxic environmental ammonia and how this sensing is translated into outputs that regulate gene expression to avoid toxicity. We found a previously unknown bacterial two-component system composed of GrtK and GrtR responsible for sensing and neutralizing toxic environmental ammonia. Understanding how pathogenic bacteria modify their toxic environment for survival can aid in the development of appropriate treatments and provide drug targets to control pathogens. Our findings suggest that GrtK and GrtR could be potential targets for drug development to control rice panicle blight caused by B. glumae.