Control of three-carbon amino acid homeostasis by promiscuous importers and exporters in Bacillus subtilis: role of the “sleeping beauty” amino acid exporters

Abstract

The Gram-positive model bacterium Bacillus subtilis can acquire amino acids by import, de novo biosynthesis, or degradation of proteins and peptides. The accumulation of several amino acids inhibits the growth of B. subtilis, probably due to misincorporation into cellular macromolecules such as proteins or peptidoglycan or due to interference with other amino acid biosynthetic pathways. Here, we studied the adaptation of B. subtilis to toxic concentrations of the three-carbon amino acids L-alanine, β-alanine, and 2,3-diaminopropionic acid, as well as the two-carbon amino acid glycine. Resistance to the non-proteinogenic amino acid β-alanine, which is a precursor for coenzyme A biosynthesis, is achieved by mutations that either activate a cryptic amino acid exporter, AexA (previously YdeD), or inactivate the amino acid importers AimA, AimB (previously YbxG), and BcaP. The aexA gene is very poorly expressed under most conditions studied. However, mutations affecting the transcription factor AerA (previously YdeC) can result in strong constitutive aexA expression. AexA is the first characterized member of a group of amino acid exporters in B. subtilis, which are all very poorly expressed. Therefore, we suggest to call this group “sleeping beauty amino acid exporters.” 2,3-Diaminopropionic acid can also be exported by AexA, and this amino acid also seems to be a natural substrate of AerA/AexA, as it can cause a slight but significant induction of aexA expression, and AexA also provides some natural resistance toward 2,3-diaminopropionic acid. Moreover, our work shows how low-specificity amino acid transporters contribute to amino acid homeostasis in B. subtilis.