How To Deal with Toxic Amino Acids: the Bipartite AzlCD Complex Exports Histidine in Bacillus subtilis

The Gram-positive model bacterium Bacillus subtilis can use several amino acids as sources of carbon and nitrogen. However, some amino acids inhibit the growth of this bacterium. This amino acid toxicity is often enhanced in strains lacking the second messenger cyclic dimeric adenosine 3',5'-monophosphate (c-di-AMP). We observed that the presence of histidine is also toxic for a B. subtilis strain that lacks all three c-di-AMP synthesizing enzymes. However, suppressor mutants emerged, and whole-genome sequencing revealed mutations in the azlB gene that encode the repressor of the azl operon. This operon encodes an exporter and an importer for branched-chain amino acids. The suppressor mutations result in an overexpression of the azl operon. Deletion of the azlCD genes encoding the branched-chain amino acid exporter restored the toxicity of histidine, indicating that this exporter is required for histidine export and for resistance to otherwise toxic levels of the amino acid. The higher abundance of the amino acid exporter AzlCD increased the extracellular concentration of histidine, thus confirming the new function of AzlCD as a histidine exporter. Unexpectedly, the AzlB-mediated repression of the operon remains active even in the presence of amino acids, suggesting that the expression of the azl operon requires the mutational inactivation of AzlB. IMPORTANCE Amino acids are building blocks for protein biosynthesis in each living cell. However, due to their reactivity and the similarity between several amino acids, they may also be involved in harmful reactions or in noncognate interactions and thus may be toxic. Bacillus subtilis can deal with otherwise toxic histidine by overexpressing the bipartite amino acid exporter AzlCD. Although encoded in an operon that also contains a gene for an amino acid importer, the corresponding genes are not expressed, irrespective of the availability of amino acids in the medium. This suggests that the azl operon is a last resort by which to deal with histidine stress that can be expressed due to the mutational inactivation of the cognate repressor AzlB.