This region of unknown function is found in RelA and SpoT of Escherichia coli, and their homologues in plants and in other eubacteria. RelA is a guanosine 3',5'-bis-pyrophosphate (ppGpp) synthetase (EC:2.7.6.5) while SpoT is thought to be a bifunctional enzyme catalysing both ppGpp synthesis and degradation (ppGpp 3'-pyrophosphohydrolase, (EC:3.1.7.2)) [1]. This region is often found in association with HD (Pfam:PF01966), a metal-dependent phosphohydrolase, TGS (Pfam:PF02824) which is a possible nucleotide-binding region, and the ACT regulatory domain (Pfam:PF01842).

1: Residual guanosine 3',5'-bispyrophosphate synthetic activity of relA null mutants can be eliminated by spoT null mutations. Xiao H, Kalman M, Ikehara K, Zemel S, Glaser G, Cashel M; J Biol Chem 1991;266:5980-5990. PMID:2005134