Role of Mediator complex in DNA damage responses.

MARISOL GIUSTOZZI; A. JASKOLOWSKI; P. CERDÁN; PAULA CASATI

Congreso; Joint LV Reunión SAIB and XIV PABMB Congress; 2019

The levels of UV-B radiation that reach the earth cause damage to proteins, lipids, RNA and DNA, inducing different responses in plants. Because of their sessile lifestyle, plants have developed defense mechanisms against this damage. One of the response pathways to DNA damage in plants involves the activation of two different protein kinases: Ataxia Telangiectasia Mutated (ATM; which mostly recognizes double strand breaks in the DNA); and Ataxia Telangiectasia mutated and RAD3-related (ATR; which mostly recognizes single strand damage and DNA crosslinks). Both kinases can activate by phosphorylation Suppressor of Gamma Response 1 (SOG1), which is a transcription factor that regulates the expression of genes involved in cell cycle transitions, DNA repair and chromatin structure. On the other hand, the Mediator complex functions as a molecular bridge between transcriptional activators and the DNA-bound RNA polymerase II. Interestingly, several reports have demonstrated that the yeast Mediator complex also acts as a regulator of DNA repair. In our lab, we recently demonstrated that A. thaliana med17 mutants also show higher DNA damage after UV-B exposure in the absence of photoreactivation than wild type plants, and that UV-B exposure to roots induces a lower accumulation of dead meristematic cells in med17 seedlings than WT plants.In this work, we analyzed the role of MED17 in the DNA damage response and its possible interaction with ATM and ATR in A .thaliana. For this purpose, we generated crosses between med17 and atr mutants, and we analyzed the effect of UV-B in med17 x atr double mutants, including programmed cell death in the meristem of primary roots after exposure to UV-B radiation (2 W.m-2). The primary root from med17 x atr mutants show a similar number of dead cells in the meristematic zone as med17 mutants, but both lines have a lower number of dead cells in the primary roots than atr mutants after exposure. We also analyzed the primary root meristem of WT, single and double mutants after UV-B exposure. med17 and med17 x atr roots show a higher decrease in the meristematic zone length by UV-B due to a higher decrease in the number of cells in the meristematic zone than WT primary root, while the decrease in the meristematic zone length in the primary roots from atr mutants by UV-B is significantly higher than the decrease measured in the other mutants. In addition, we compared the fertility of med17 x atr, med17 and atr mutants. med17 x atr mutants have a higher percentage of aborted seeds than Col-0 and atr mutants, but similar to those in med17. Together, our results suggest that med17 is epistatic over atr.