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The Effects of UV-A Radiation on Circadian Rhythms in Synechococcus Elongatus UTEX 2973

Anh Nguyen

Research on physiological activities of cyanobacteria has indicated the presence of circadian rhythms synchronized with a 24-hour, light-dark cycle of the external environment. In Synechococcus elongatus PCC 7942, the input pathway relays information to the output pathway via the circadian oscillator encoded by KaiABC proteins. While redox-sensitive signaling proteins such as CikA (circadian input kinase A) and LdpA (light-dependent period A) regulate the input pathway, the output pathway consisting of SasA (Synechococcus adaptive sensor A) and RpaA (regulator of phycobilisome association A) controls the timing of gene expression and cell division. Despite extensive studies on circadian activities in cyanobacteria and their dependence on light energy as photoautotrophs, little is known about the effects of ultraviolet (UV) radiation on circadian rhythms in cyanobacteria. The objective of this study is to evaluate the effects of long-wavelength UV-A radiation on circadian rhythms in the fastest growing cyanobacterium known to date, Synechococcus elongatus UTEX 2973. This organism was grown in a diel (12hours:12hours), light-dark cycle either with white light only or supplemented with UV-A during the light period over 72 hours. Samples taken every 6 hours were used to assess growth patterns. A consistent increase in growth during the light period was observed for both white light and UV-A treatment groups, although the white light group showed faster growth rates. Samples were also taken every 12 hours for 48 hours to measure the expression of genes associated with the circadian system (kaiABC, cikA, ldpA, sasA, and rpaA) using quantitative-PCR. Exposure to UV-A negatively affected kaiA and kaiB expression, as the genes were downregulated during the first light period, while kaiC upregulation during the last dark period indicated a contrary effect. The differential changes seen for kaiC could be because KaiC protein activity relies on post-translational phosphorylation rather than gene expression. It is also possible that gene expression and protein activity occur during opposite times of the cycle to allow for the lag between transcription and a functional protein. Among the regulatory genes, UV-A radiation did not yield a significant change in cikA, ldpA, and sasA expression over time. However, rpaA expression was generally upregulated over the course of the experiment except after the first 12-hour light period in the UV-A treatment group. RpaA protein is considered a “master regulator” of the circadian output system, so it is not surprising to see an enhanced response in the expression of this gene compared to the others. To further investigate the impacts of the circadian oscillator on growth and gene expression, a kaiABC mutant will be generated using CRISPR genome editing. The results of this study will provide a baseline understanding of the effects of UV-A on circadian systems in cyanobacteria and offer insight toward the development of future studies.

2022

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