ppGpp Controls Global Gene Expression in Light and in Darkness in S. elongatus.

The bacterial and plant stringent response involves production of the signaling molecules guanosine tetraphosphate and guanosine pentaphosphate ((p)ppGpp), leading to global reorganization of gene expression. The function of the stringent response has been well characterized in stress conditions, but its regulatory role during unstressed growth is less studied. Here, we demonstrate that (p)ppGpp-deficient strains of S. elongatus have globally deregulated biosynthetic capacity, with increased transcription rate, translation rate, and cell size in unstressed conditions in light and impaired viability in darkness. Synthetic restoration of basal guanosine tetraphosphate (ppGpp) levels is sufficient to recover transcriptional balance and appropriate cell size in light and to rescue viability in light/dark conditions, but it is insufficient to enable efficient dark-induced transcriptional shutdown. Our work underscores the importance of basal ppGpp signaling for regulation of cyanobacterial physiology in the absence of stress and for viability in energy-limiting conditions, highlighting that basal (p)ppGpp level is essential in cyanobacteria in the environmental light/dark cycle.