Signalling by the global regulatory molecule ppGpp in bacteria and chloroplasts of land plants

The hyperphosphorylated guanine ribonucleotide ppGpp mediates the stringent response in bacteria. Biochemical and genetic studies of this response in Escherichia coli have shown that the biosynthesis of ppGpp is catalysed by two homologous enzymes, RelA and SpoT. RelA is activated in response to amino acid starvation, and SpoT responds to abiotic physical stress beside nutritional stress. All free-living bacteria, including Gram-positive firmicutes, contain RelA-SpoT homologues (RSH). Further, novel ppGpp biosynthetic enzymes, designated small alarmone synthetases (SASs), were recently identified in a subset of bacteria, including the Gram-positive organism Bacillus subtilis, and were shown to consist only of a ppGpp synthetase domain. Studies suggest that these SAS proteins contribute to ppGpp signalling in response to stressful conditions in a manner distinct from that of RelA-SpoT enzymes. SAS proteins currently appear to always occur in addition to RSH enzymes in various combinations but never alone. RSHs have also been identified in chloroplasts, organelles of photosynthetic eukaryotes that originated from endosymbiotic photosynthetic bacteria. These chloroplast RSHs are exclusively encoded in nuclear DNA and targeted into chloroplasts. The findings suggest that ppGpp may regulate chloroplast functions similar to those regulated in bacteria, including transcription and translation. In addition, a novel ppGpp synthetase that is regulated by Ca²⁺ as a result of the presence of two EF-hand motifs at its COOH terminus was recently identified in chloroplasts of land plants. This finding indicates the existence of a direct connection between eukaryotic Ca²⁺ signalling and prokaryotic ppGpp signalling in chloroplasts. The new observations with regard to ppGpp signalling in land plants suggest that such signalling contributes to the regulation of a wider range of cellular functions than previously anticipated.