Regulation of cyanobacterial physiology by the stringent response
Signals that communicate a cell’s metabolic state allow organisms to adapt to the suboptimal conditions they often encounter in the environment. If a particular nutrient is lacking, the organism reduces the energy it expends on growth and reproduction, instead redirecting its resources to maintain basic cellular functions.
Cyanobacteria were the driving force behind the oxygenation of Earth’s atmosphere, the ancestor of the chloroplast, and remain an important group of primary producers today. These ‘blue-green’ bacteria carry out photosynthesis, capturing solar light energy and converting it into the chemical energy that sustains life. They also perform carbon fixation, reducing carbon dioxide into organic carbon compounds. These processes are both required to sustain cyanobacterial growth, so when the inputs of these pathways – like light – are unavailable, many metabolic changes occur that necessitate physiological adaptations.
Here, I demonstrate a major mechanism by which the cyanobacterium Synechococcus elongatus senses and responds to stresses. I find that the stringent response, a conserved bacterial stress response, is induced in response to darkness, photosynthetic inhibition, and nutrient starvation. The second messengers of this response, the phosphorylated nucleotides ppGpp and pppGpp, reprogram gene expression and induce physiological responses that can be either general or nutrient-specific. I have also begun investigating the roles of polyphosphate, a compound that accumulates in response to stress and may help cells adapt to challenging conditions, in Synechococcus.
Cyanobacteria like Synechococcus are an interesting subject for study of the stringent response due to their unique metabolism and physiology. While the logic of the stringent response – indeed, many stress responses – is similar across many bacteria, the specific mechanisms behind them often differ. Learning more about how the same stress response mechanism has been adapted by diverse organisms will continue to increase our understanding of how bacteria interact with their environments.