Abstract
The ppGpp mediates the stringent response in bacteria. In Escherichia coli, the biosynthesis of ppGpp is catalyzed by two homologous enzymes, RelA and SpoT. RelA-SpoT homologs (RSHs) have also been identified in chloroplasts, suggesting that ppGpp may regulate chloroplast functions similar to those regulated in bacteria. Recently, a novel (p)ppGpp synthetase that is regulated by Ca2+ as a result of the presence of EF-hand motifs at its C-terminus was identified in chloroplasts of land plants. In addition, novel ppGpp biosynthetic enzymes, designated small alarmone synthetases (SASs), were also identified in the Gram-positive organism Bacillus subtilis, and were shown to consist only of a ppGpp synthetase domain. These new observations with regard to ppGpp signaling in both bacteria and land plants have suggested that such signaling contributes to the regulation of a wider range of cellular functions than previously anticipated.
