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. 2022 Dec 5;19(3):334–345. doi: 10.1038/s41589-022-01198-x

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© The Author(s) 2022

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. 1 — a, Evolution of long RSHs in Proteobacteria. Duplication of the ancestral bifunctional RSH Rel in Beta- and Gammaproteobacterial lineages gave rise to RelA and SpoT, leading to subfunctionalization of RelA as monofunctional SYNTH-only alarmone synthetase and SpoT as a predominantly HD RSH. In the Moraxellaceae family of Gammaproteobacteria, SpoT has undergone further subfunctionalization, evolving into a monofunctional HD-only alarmone HD. b, Alignment of SYNTH-critical regions in long RSHs highlights the sequence divergence in Moraxellaceae SpoTs. c, Coexpression of SpoT_Ab counteracts the growth defect in ppGpp⁰ (ΔrelA ΔspoT) E. coli caused by RelA expression. This demonstrates that SpoT_Ab is HD active in the E. coli host. d, While the SYNTH activity of ectopically expressed SpoT_Ec is essential and sufficient for promoting the growth of ppGpp⁰ E. coli on M9 minimal medium, SpoT_Ab fails to promote the growth of ΔrelA ΔspoT E. coli on M9. This demonstrates that, unlike SpoT_Ec, which is SYNTH-active, SpoT_Ab is SYNTH-inactive.