ACT domain found C-terminal of the RelA/SpoT domains
ACT_RelA-SpoT: the ACT domain found C-terminal of the RelA/SpoT domains. Enzymes of the Rel/Spo family enable bacteria to survive prolonged periods of nutrient limitation by controlling guanosine-3'-diphosphate-5'-(tri)diphosphate ((p)ppGpp) production and subsequent rRNA repression (stringent response). Both the synthesis of (p)ppGpp from ATP and GDP(GTP), and its hydrolysis to GDP(GTP) and pyrophosphate, are catalyzed by Rel/Spo proteins. In Escherichia coli and its close relatives, the metabolism of (p)ppGpp is governed by two homologous proteins, RelA and SpoT. The RelA protein catalyzes (p)ppGpp synthesis in a reaction requiring its binding to ribosomes bearing codon-specified uncharged tRNA. The major role of the SpoT protein is the breakdown of (p)ppGpp by a manganese-dependent (p)ppGpp pyrophosphohydrolase activity. Although the stringent response appears to be tightly regulated by these two enzymes in E. coli, a bifunctional Rel/Spo protein has been discovered in most gram-positive organisms studied so far. These bifunctional Rel/Spo homologs (rsh) appear to modulate (p)ppGpp levels through two distinct active sites that are controlled by a reciprocal regulatory mechanism ensuring inverse coupling of opposing activities. In studies with the Streptococcus equisimilis Rel/Spo homolog, the C-terminal domain appears to be involved in this reciprocal regulation of the two opposing catalytic activities present in the N-terminal domain, ensuring that both synthesis and degradation activities are not coinduced. Members of this CD belong to the superfamily of ACT regulatory domains.