Stringent response is run by the RSH (RelA / SpoT Homologue) proteins, but there are more RSHs then just these two. Usually researchers were finding them using an ad hoc approach: take your favorite bug you worked with for 10 years, blast its genome with RelA gene, find anything that looks like RelA, test it.
Finally there is a proper analysis of RSHs across the tree of life: The RelA/SpoT Homolog superfamily: distribution and functional evolution of ppGpp synthetases and hydrolases across the tree of life by Atkinson GC, Tenson T, Hauryliuk V, PLoS ONE, 6(8): e23479.
Here is the take home message:
- there are loads of different RSHs out there: we identified 30 subgroups!
- all the RSHs out there are now are classified (for now, that is. New genomes are coming out every day, damn the progress!).
- Archaea, Bacteria, Eucaryotes: they all have RSHs. I repeat: Archaea too.
- there are the long RSHs (i.e. Rel, RelA and SpoT) and there are the short ones.
- The short ones have either ppGpp synthesis or ppGpp hydrolysis domain. The long ones have both, but not always both are functional.
- by comparing the long ones vs the short ones we identified residues potentially involved in the inter-domain cross-talk in the long ones (the short ones have only one domain thus there is no cross talk there!).
The bottom line: if you work on a strange RSH protein from a strange bug, do check out our paper.
Hurray to Gem!