Mechanism Of RTEL1 On Regulating Transcriptional Gene Silencing And DNA Repair In Arabidopsis

Epigenetic regulation refers to a phenomenon that influences gene transcription through changing the modifications of DNA and histone or the higher-order chromatin organization beyond the genomic DNA sequence.Epigenetic regulation of gene silencing includes transcriptional gene silencing(TGS)and post transcriptional gene silencing(PTGS).TGS is mediated by different pathways,one of these pathways is the DNA replication and repair-mediated TGS(DRR-TGS).To identify genes involving in TGS,we performed a genetic screen using EMS-mutated seeds of the L119 transgenic line,which carries a T-DNA locus with silenced ProRD29A:LUC and Pro35S:NPT?.We isolated a human homolog RTEL1(Regulator of telomere length 1)gene encoding a DNA helicase which is involved in DNA replication and repair.NPTII but not LUC is reactivated in rtell-3.RTEL1 regulates NPTII independent of DNA methylation,but depend on the level of histone H3.Treatment with DNA-damaging agents activates the silenced gene NPTII in L119,suggesting that NPT? expression is modulated by DNA damage.However,the RTEL1 deficiency mimics a DNA damage status caused by cross-linked DNA,so the activation of NPT? in rtell-3 may associate with the damage status.Collectively,we demonstrate that RTEL1 is a new factor that is involved in the DRR-TGS pathway.rtell-3 mutant possesses the phenotype of shorter roots and reduced DNA replication activity in root meristem compared to L119,indicating that RTEL1 participates in DNA replication in Arabidopsis.rtell-3 is sensitive to treatments of MMS and Cisplatin,but shows resistance to HU.Besides,rtell-3 mutant is supersensitive to the DNA-damaging and methylation-inhibiting agent zebularine,whereas rtell-3 shows no obvious sensitivity to RG108 which is a inhibitor of DNA methylation,demonstrating that the supersensitivity to zebularine is due to the DNA damage but not hypomethylation.DDM1 and MET1 play important roles in maintaining DNA methylation,and DDM1 is responsible for DNA repair according to previous reports.DNA methylation is significantly reduced in ddm1-101 and metl-101 mutants.In our study,we find that ddml-101 and metl-101 are sensitive to zebularine and DNA-damaging agents,and the double mutants rtell-3 ddml-101 and rtell-3 metl-101 show severe growth defects.However,double mutants of rtell-3 and the RdDM components do not have the similar additive phenotypes,indicating that the phenotypes of rtell-3 ddml-101 and rtell-3 met1-101 reflect defects in DNA damage rather than DNA methylation.The findings above suggest that RTEL1 and DDM1/MET1 act synergistically to modulate DNA repair pathway and maintain the genomic integrity.