Despite the environmental and biotechnological importance of cyanobacteria, most of the molecular mechanisms involved in maintaining cellular homeostasis and adaptation to environmental changes remain unknown or hardly studied in comparison to other bacterial groups. This is the case of signalling by (p) ppGpp (guanosine penta- and tetra-phosphate), a complex process involving the reorganization of gene expression, which is conserved in bacteria and plants. We show here that some levels of (p) ppGpp are required for S. elongatus viability under standard laboratory growth conditions and exploited this finding to identify suppressor mutations partially restoring viability. We found that, as in firmicutes, the purine biosynthesis pathway appears to be the main target of (p) ppGpp signalling. In addition, the translation machinery also emerged as a key cyanobacterial target, a result facilitated by the occurrence of a small inversion targeting residues 15His-16Phe at the essential ribosomal subunit S2. Finally, the identification of a putative nucleotide signaling protein, encoded by synpcc7942₀187, points to regulatory peculiarities involved in (p) ppGpp signalling in cyanobacteria. This work paves the way for a molecular understanding of the functions and targets of (p) ppGpp in cyanobacteria, anticipating novel regulatory mechanisms unique to this important and understudied bacterial group.
Llop et al. (Wed,) studied this question.