hide
Free keywords:
-
Abstract:
Cyanobacteria are phototrophic prokaryotes that evolved oxygenic photosynthesis ∼2.7 billion y ago. For efficient CO 2 fixation at low ambient concentrations, they evolved highly specialized carbon concentrating mechanism that employs high affinity inorganic carbon (Ci) uptake systems, to augment intracellular Ci levels 1 . Due to this unique photosynthetic lifestyle, they evolved as well sophisticated regulatory mechanisms to adapt to oscillating day-night metabolic changes between autotrophic and heterotrophic metabolisms. Recently, we identified the PII-like protein SbtB as Ci sensing module2 . Similar to nitrogen-regulatory PII proteins, SbtB is able to bind the adenine nucleotides ATP and ADP, but unlike these, it also binds AMP and moreover, it preferentially binds the second messengers cAMP2 and c-di-AMP 3 . We showed that cAMP acts as carbon signal, whereas adenyl-nucleotide binding may link SbtB signalling to the energy state of the cells. In search for a function for SbtB as c-di-AMP receptor, we found that both of SbtB and c-di-AMP cyclase knockout mutants were impaired under diurnal growth. Moreover, SbtB was identified as controlling factor for glycogen metabolism through interaction with the glycogen branching enzyme GlgB. Thus, c-di-AMP signaling through SbtB turned out pivotal for day-night acclimation of cyanobacteria via regulation of glycogen metabolism. To our knowledge, this is the first signaling protein known to sense both cAMP and c-di-AMP. This highlights the central role of SbtB as a switch point in cyanobacterial cell physiology, integrating not only signals from the energy state and carbon supply through adenine-nucleotide and cAMP binding, respectively, but also from the diurnal state by binding to c-di-AMP.
