Temperature effects on UV-B induced DNA damage and repair in plants and a lichen
Abstract: All living organisms have DNA and since DNA is essential to life it must be repaired when damaged. UV radiation is one of the major DNA damaging agents, which photosynthetic organisms cannot avoid. The major UV-induced DNA lesions are cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs) and the main repair pathways of theses lesions involve photolyases and various excision repair systems. Photolyases require longer wavelength UV-A or blue light to induce repair. This thesis summarises a series of investigations on the effect of temperature (0-30°C) on induction of UV-B induced DNA damage, the photorepair of those UV-lesions and on the activity of purified photolyase in tobacco (Nicotiana tabacum) BY-2 cells and leaves, lingonberry (Vaccinium vitis-idaea) leaves, and Cladonia arbuscula ssp. mitis thalli. Several studies described here show that the induction of CPDs and 6-4PPs is temperature dependent, with more lesions at higher temperatures. There is variation between species with the most remarkable being that almost no induction of CPDs was detectable in lingonberry at 0°C after 60 minutes of UV-B radiation. Greenhouse grown tobacco showed larger induction of CPDs than naturally growing lingonberry under the same temperature and light conditions. The temperature dependence of the subsequent photorepair in vivo was measured using two different approaches. In the first approach samples were irradiated with UV-B at a given temperature resulting in a constant amount of lesions at the start of the subsequent photorepair experiment under varying temperatures. In the second approach the temperature utilised for induction of damage was the same as that of the following repair, resulting in varying amount of lesions at the start of photorepair. More lesions were generally repaired at higher temperatures than at lower. However, the repair was more efficient at lower temperatures in lingonberry, compared to tobacco leaves. Measurements of CPD-photolyase activity in vitro from tobacco and lingonberry showed little response to temperature increases, indicating that the photochemical reaction is rate-limiting under the conditions used here. The temperature effects on induction and repair in vivo are suggested to be mainly due to effects on the interaction between DNA and other cellular components such as the nucleosomes and the transcriptional apparatus.
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