Second messengers are intracellular substances regulated by specific external stimuli globally known as first messengers. Cells rely on second messengers to generate rapid responses to environmental changes and the importance of their roles is becoming increasingly realized in cellular signaling research. Cyanobacteria are photooxygenic bacteria that inhabit most of Earth's environments. The ability of cyanobacteria to survive in ecologically diverse habitats is due to their capacity to adapt and respond to environmental changes. Chapter 1 will review known second messenger-controlled physiological processes in cyanobacteria. Second messengers used in these systems include the element calcium (Ca2+), the nucleotide-based guanosine tetraphosphate or pentaphosphate (ppGpp or pppGpp, represented as (p)ppGpp), the cyclic adenosine 3′,5′-monophosphate (cAMP), the c-di-GMP, the cyclic guanosine 3′,5′-monophosphate (cGMP), and the cyclic dimeric AMP (c-di-AMP), and the gaseous nitric oxide (NO). The discussion will focus on processes central to cyanobacteria such as nitrogen fixation, light perception, photosynthesis-related processes, and gliding motility. Chapter 2 will confirm that many putative c-di-GMP synthesis or degradation domains are found in genes that also harbor light-responsive signal input domains, suggesting that light is an important signal for altering c-di-GMP homeostasis. I will also compare intracellular levels of c-di-GMP in Synechocystis sp. PCC 6803 and Fremyella diplosiphon under different light qualities, confirming that light is an important factor for regulating this second messenger in vivo. Chapter 3 will demonstrate that biofilm formation, cellular aggregation or flocculation, and cellularbuoyancy are under the control of c-di-GMP in Synechocystis sp. PCC 6803 and F. diplosiphon. In this chapter, I present data on Synechocystis and F. diplosiphon transformed with a plasmid for constitutive expression of genes encoding diguanylate cylase (DGC) and phosphodiesterase (PDE) proteins from Vibrio cholerae and Escherichia coli, respectively. Engineering of these strains allowed modulation of intracellular c-di-GMP levels. Chapter 4 will confirm the role of c-di-GMP in F. diplosiphon in regulating pigments. I show that c-di-GMP is involved in light signalling networks and correctly tunes the pigments of cells under changes to ambient light. In chapter 5, I conclude by addressing future research trajectories needed to better understand the signaling networks and cross talk in the signaling pathways of these molecules in cyanobacteria. Second messengers have significant potential to be adapted as technological tools and possible novel and practical applications are highlighted based on current understanding of these molecules and the signaling networks that they control.
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