Guanosine tetraphosphate and pentaphosphate, collectively known as (p)ppGpp or magic spot nucleotides, are effectors of the stringent response, a stress response pathway activated during nutrient and energy deprivation. The stringent response is conserved in bacteria, algae and plants and is characterized by a rapid accumulation of (p)ppGpp that leads to global transcriptional reprogramming, shifting gene expression from genes responsible for growth, housekeeping and division towards genes responsible for stress protection and cell survival. While it is widely acknowledged that elevated levels of ppGpp reorganize gene expression profiles of various organisms during adaptation to adverse conditions, our understanding of the role of ppGpp signaling in regulation of cell physiology in unstressed conditions has been more limited.
In this study (PDF), we use the cyanobacterium Synechococcus elongatus to gain insight into the function of ppGpp during unstressed growth in light and in response to darkness, which is equivalent in cyanobacteria to a period of energy-limitation and stress. It has been observed previously that in cyanobacteria (p)ppGpp accumulates on exposure to darkness, however, its role during growth in light has not been investigated.
We find that ppGpp is present in cyanobacteria at a basal low level in light in the absence of stress and rapidly accumulates in response to inhibition of the activity of the photosynthetic electron transport chain. We next demonstrate that basal levels of ppGpp are key for regulation of normal cell physiology in unstressed conditions in light, as the ppGpp-deficient strain suffers from a global transcriptional increase, elevated global transcription and translation rates and increased cell size. In the dark, we find that our ppGpp-deficient strain is unable to appropriately execute normal dark-induced transcriptional suppression and that it loses viability under these conditions. Finally, we dissect the role of basal versus elevated ppGpp levels in regulation of cell physiology. When we restore basal ppGpp levels in the ppGpp-deficient strain, we observe that cells recover appropriate transcription levels and cell size in light and are able to survive periods of darkness. However, the synthetically restored low level of ppGpp is not sufficient to appropriately execute the dark-induced transcriptional shutdown.
In our work we establish how (p)ppGpp signaling is used in cyanobacteria to regulate cell physiology both in light and during exposure to darkness, emphasizing the essential contribution of basal levels of ppGpp to the maintenance of cellular homeostasis. We proposed a model in which light-responsive (p)ppGpp signaling is required for appropriate adjustment of cyanobacterial transcription and metabolism in fluctuating light conditions. This dose-dependent model of (p)ppGpp signaling is in accordance with observations made in Firmicutes, in which the magic spot nucleotides have been proposed to act as a molecular rheostat tuning cell physiology to environmental constraints.