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. 2020 Oct 9;11:563944. doi: 10.3389/fmicb.2020.563944

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Copyright © 2020 Das and Bhadra.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Schematic representation of the domain organization of (p)ppGpp synthetase/hydrolase family of proteins. Bifunctional long multidomain (p)ppGpp synthetase-hydrolase and monofunctional long multidomain (p)ppGpp synthetase enzymes are composed of two regions: (i) N-terminal catalytic domain (CD) and (ii) C-terminal regulatory domain (RD). Both synthetase and hydrolase domains (SD and HD, respectively) are functional in bifunctional enzymes (Rel/RSH/SpoT). In monofunctional multidomain, (p)ppGpp synthetase enzyme (RelA) hydrolase activity is lost due to accumulation of spontaneous mutations in the catalytic residues. The C-terminal regulatory region of the multidomain protein contains TGS domain (ThrRS, GTPase, and SpoT), conserved α helical domain, ZFD domain (zinc-finger or conserved cysteine domain), and ACT (aspartate kinase, chorismate, and TyrA) RNA recognition motif. The small alarmone synthetase (SAS) enzyme (RelV/RelP/RelQ) contains SD and a small oligomeric α-domain. The RelH/small alarmone hydrolase (SAH) contains only the HD.