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. 2021 Feb 23;12(1):e03020-20. doi: 10.1128/mBio.03020-20

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Copyright © 2021 Zhou et al.

This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license.

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FIG 5 — Tryptophan directly binds GlnD and GlnE to trigger (p)ppGpp accumulation. (A and B) Surface plasmon resonance (SPR) experiments show that tryptophan binds GlnD and GlnE. Purified GlnD or GlnE was tethered to the SPR chip and exposed to increasing concentrations of tryptophan. The binding period is displayed as response units plotted as function of time, followed by buffer injection to remove free tryptophan. Increasing concentrations of tryptophan used in the SPR experiments are shown. (C) Tryptophan stimulates (p)ppGpp accumulation. (Left) Intracellular levels of (p)ppGpp were detected by TLC. Alanine serves as a negative control. (Right) Relative concentrations of ppGpp as a function of tryptophan availability are shown. Mean and SD are plotted (n = 4). An asterisk indicates a significant difference (P < 0.05) by two-tailed Student’s t test in comparison with tryptophan-treated wild-type cells.