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. 2020 Mar 5;11:1203. doi: 10.1038/s41467-020-15017-1

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 7 — Free methionine can enter to the cell by two ways, via a low-affinity BcaP-transporter (branched-chain amino acid permease) and through a high-affinity Met-transporter (composed of the PlpABCD, YdcC and YdcB proteins). The met operon is regulated by CmhR, using homocysteine (L-HC) as coeffector. The expression of bcaP is controlled by CodY, which is activated by branched-chain amino acids (BCAA). At low methionine concentrations, the cells increase methionine uptake rates to sustain growth and two colony phenotypes are observed: GFP− colonies (a; left) and GFP+ colonies (b; right). a In GFP− colonies the repression of bcaP by CodY is released timely, and the upregulation of the low-affinity transporter results in enough methionine to support growth. Although GFP− colonies weakly express the Met transporter as well. b In GFP+ colonies, methionine starvation leads to the presence of uncharged tRNA^Met, and this signal strongly up-regulates the expression of the met operon via the regulatory element (RE; T-box riboswitch) in the leader region of the met mRNA, which assures transcriptional continuation. In addition, the uncharged tRNA^Met triggers the stringent response due to amino acid starvation.