AUTHOR=Agostoni Marco , Logan-Jackson Alshaé R. , Heinz Emily R. , Severin Geoffrey B. , Bruger Eric L. , Waters Christopher M. , Montgomery Beronda L. 

TITLE=Homeostasis of Second Messenger Cyclic-di-AMP Is Critical for Cyanobacterial Fitness and Acclimation to Abiotic Stress

JOURNAL=Frontiers in Microbiology

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2018.01121

DOI=10.3389/fmicb.2018.01121

ISSN=1664-302X

ABSTRACT=<p>Second messengers are intracellular molecules regulated by external stimuli known as first messengers that are used for rapid organismal responses to dynamic environmental changes. Cyclic di-AMP (c-di-AMP) is a relatively newly discovered second messenger implicated in cell wall homeostasis in many pathogenic bacteria. C-di-AMP is synthesized from ATP by diadenylyl cyclases (DAC) and degraded by specific c-di-AMP phosphodiesterases (PDE). C-di-AMP DACs and PDEs are present in all sequenced cyanobacteria, suggesting roles for c-di-AMP in the physiology and/or development of these organisms. Despite conservation of these genes across numerous cyanobacteria, the functional roles of c-di-AMP in cyanobacteria have not been well-investigated. In a unique feature of cyanobacteria, phylogenetic analysis indicated that the broadly conserved DAC, related to CdaA/DacA, is always co-associated in an operon with genes critical for controlling cell wall synthesis. To investigate phenotypes regulated by c-di-AMP in cyanobacteria, we overexpressed native DAC (<italic>sll0505</italic>) and c-di-AMP PDE (<italic>slr0104</italic>) genes in the cyanobacterium <italic>Synechocystis</italic> sp. PCC 6803 (hereafter <italic>Synechocystis</italic>) to increase and decrease intracellular c-di-AMP levels, respectively. DAC- and PDE-overexpression strains, showed abnormal aggregation phenotypes, suggesting functional roles for regulating c-di-AMP homeostasis <italic>in vivo</italic>. As c-di-AMP may be implicated in osmotic responses in cyanobacteria, we tested whether sorbitol and NaCl stresses impacted expression of <italic>sll0505</italic> and <italic>slr0104</italic> or intracellular c-di-AMP levels in <italic>Synechocystis</italic>. Additionally, to determine the range of cyanobacteria in which c-di-AMP may function, we assessed c-di-AMP levels in two unicellular cyanobacteria, i.e., <italic>Synechocystis</italic> and <italic>Synechococcus elongatus</italic> PCC 7942, and two filamentous cyanobacteria, i.e., <italic>Fremyella diplosiphon</italic> and <italic>Anabaena</italic> sp. PCC 7120. C-di-AMP levels responded differently to abiotic stress signals in distinct cyanobacteria strains, whereas salt stress uniformly impacted another second messenger cyclic di-GMP in cyanobacteria. Together, these results suggest regulation of c-di-AMP homeostasis in cyanobacteria and implicate a role for the second messenger in maintaining cellular fitness in response to abiotic stress.</p>