On Clocks and Tuners: Cyanobacterial Strategies to Thrive in a Dynamic Environment

Abstract

The cycle of day and night is among the most recurrent and predictable environmental changes. In order to thrive, organisms across all domains of life evolved regulatory strategies to respond to diurnal fluctuations in light availability. Here, we utilize the cyanobacterium Synechococcus elongatus PCC7942 to gain insight into the role of the circadian and light- responsive systems in regulation of cyanobacterial physiology in conditions mimicking the day and night cycle. In Chapter 2, we explore the function of the cyanobacterial circadian program. We utilize the known viability defect of a mutant lacking circadian oscillations in gene expression (the rpaA− strain) to understand how the circadian system schedules biological processes to give an organism an ability to better survive diurnal changes in light availability. We demonstrate that during the day the rpaA− strain is unable to prepare carbohydrate stores required for darkness and that it does not express glycogen-utilizing enzymes at dusk. These defects lead to an inability to correctly maintain internal energy levels in the dark in this strain. Our work highlights that one important function of the circadian system in S. elongatus is coordination of carbohydrate metabolism with the cycle of day and night. In Chapter 3, we study the role of the signaling nucleotide (p)ppGpp in regulation of cyanobacterial physiology in light and in the dark. We show that basal ppGpp levels are required for appropriate control of global transcription, translation and cell size in light, and are essential for cell viability in the dark. Elevated ppGpp levels, on the other hand, are necessary for appropriate execution of the dark-induced transcriptional shutdown. Our study demonstrates that light-responsive (p)ppGpp signaling is crucial for control of normal physiology in cyanobacteria. Together, our work highlights how circadian and light-responsive programs coordinate different aspects of cell physiology to bring about adaptive advantage in periodically changing environments.