Understanding the UV-B response and the role of nitric oxide in the chromatically adapting cyanobacterium Synechococcus PCC 7335

Solar ultraviolet-B (UV-B) radiation exerts different responses in organisms based on the exposure dose and duration. Those with autotrophic lifestyles are continuously exposed to sunlight and evolved mitigation strategies. Nitric oxide (NO) is a gaseous signaling molecule associated with the protection towards UV-B in plants and cyanobacteria. Synechococcus PCC 7335 is a peculiar cyanobacterium with adaptations to different light regimes and encodes for a non-canonical nitric oxide synthase (NOS). In this study, we analyzed the effects of UV-B and the role of NO during this response. Our findings demonstrate that UV-B treatment slows growth recovery although the culture remains viable. Also, NO levels increase post-irradiation through a mechanism independent of NOS and nitrate reductase (NR), suggesting a probable increase via a non-enzymatic pathway. In addition, UV-B reduces phycobiliprotein (PBP) content, but has no effect on chlorophyll a level. Arginine addition, which is the NOS substrate, has no protective effect towards pigment degradation triggered by UV-B. At the molecular level, UV-B induces the expression of the ObgE homolog gene along with a photolyase operon enabling the culture to counteract the radiation. The deregulated NO production is not involved in the regulation of the expression of these genes. In addition, both UV-B and exogenous NO prevent S. PCC 7335 biofilm formation. This report gives new insight into the response of cyanobacteria to UV-B radiation, which differs from those described for other cyanobacteria.