Abstract
Ultraviolet (UV) radiation can cause stresses or act as a photoregulatory signal depending on its wavelengths and fluence rates. Although the most harmful effects of UV on living cells are generally attributed to UV-B radiation, UV-A radiation can also affect many aspects of cellular processes. In cyanobacteria, most studies have concentrated on the damaging effect of UV and defense mechanisms to withstand UV stress. However, little is known about the activation mechanism of signaling components or their pathways which are implicated in the process following UV irradiation. Motile cyanobacteria use a very precise negative phototaxis signaling system to move away from high levels of solar radiation, which is an effective escape mechanism to avoid the detrimental effects of UV radiation. Recently, two different UV-A-induced signaling systems for regulating cyanobacterial phototaxis were characterized at the photophysiological and molecular levels. Here, we review the current understanding of the UV-A mediated signaling pathways in the context of the UV-A perception mechanism, early signaling components, and negative phototactic responses. In addition, increasing evidences supporting a role of pterins in response to UV radiation are discussed. We outline the effect of UV-induced cell damage, associated signaling molecules, and programmed cell death under UV-mediated oxidative stress.
Highlights
Ultraviolet light A (UV-A) (315–400 nm) comprises the largest portion of the total solar UV reaching the earth [1], since all the UV-C (~180–280 nm) and most of the extraterrestrial ultraviolet B (UV-B) (280–315 nm) are absorbed by the earth’s stratospheric ozone layer
UV-B has the greatest potential for cell damage, which is caused by both direct effects on DNA and proteins, as well as indirect effects via the production of reactive oxygen species [10,11]
This review provides a possible function of pterins in cyanobacteria, namely as a chromophore of UV-A photoreceptor systems
Summary
Ultraviolet light A (UV-A) (315–400 nm) comprises the largest portion of the total solar UV reaching the earth [1], since all the UV-C (~180–280 nm) and most of the extraterrestrial ultraviolet B (UV-B) (280–315 nm) are absorbed by the earth’s stratospheric ozone layer. Cyanobacteria, like plants and algae, have developed physiological mechanisms allowing acclimation and survival in harmful UV irradiation conditions. Their protection strategies include stress avoidance by migration, stress defense by induction of UV-absorbing molecules as well as antioxidants, and repair mechanisms including DNA repair and de novo synthesis of D1 and D2 proteins for the repair of the damaged photosystem II complex [25,26,27,28,29,30]. We discuss the effects of UV irradiation on cyanobacteria, as well as their programmed cell death to counteract UV damage in view of UV-sensing and signaling under UV-mediated oxidative stress
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