Twist and turn: a revised structural view on the unpaired bubble of class II CPD photolyase in complex with damaged DNA

Visualizing the DNA repair process by a photolyase at atomic resolution

Photolyases, a ubiquitous class of flavoproteins, use blue light to repair DNA photolesions. In this work, we determined the structural mechanism of the photolyase-catalyzed repair of a cyclobutane pyrimidine dimer (CPD) lesion using time-resolved serial femtosecond crystallography (TR-SFX). We obtained 18 snapshots that show time-dependent changes in four reaction loci. We used these results to create a movie that depicts the repair of CPD lesions in the picosecond-to-nanosecond range, followed by the recovery of the enzymatic moieties involved in catalysis, completing the formation of the fully reduced enzyme-product complex at 500 nanoseconds. Finally, back-flip intermediates of the thymine bases to reanneal the DNA were captured at 25 to 200 microseconds. Our data cover the complete molecular mechanism of a photolyase and, importantly, its chemistry and enzymatic catalysis at work across a wide timescale and at atomic resolution.

Time-resolved crystallography captures light-driven DNA repair

Photolyase is an enzyme that uses light to catalyze DNA repair. To capture the reaction intermediates involved in the enzyme's catalytic cycle, we conducted a time-resolved crystallography experiment. We found that photolyase traps the excited state of the active cofactor, flavin adenine dinucleotide (FAD), in a highly bent geometry. This excited state performs electron transfer to damaged DNA, inducing repair. We show that the repair reaction, which involves the lysis of two covalent bonds, occurs through a single-bond intermediate. The transformation of the substrate into product crowds the active site and disrupts hydrogen bonds with the enzyme, resulting in stepwise product release, with the 3' thymine ejected first, followed by the 5' base.

Electron Transfer-Induced Active Site Structural Relaxation in 64-Photolyase of Drosophila melanogaster

While catalytic electron flow and photoreactivation of CPD-photolyases are increasingly understood, the microscopic details of the 64-photolyase repair mechanism are perpetually debated. Here, we investigate in long-time (μs) molecular dynamics simulations combined with extensive quantum mechanical/molecular mechanical (QM/MM) simulations the primary electron transfer (ET) reactions in 64-photolyase of Drosophila melanogaster (D. melanogaster). The characterization of the relative energetics of locally excited and charge separated states in the (6-4) photoproduct enzyme repair complex reveals a charge-separated state involving the adenine moiety of the FADH^- cofactor that facilitates reduction of the photoproduct. Microscopic details of the collective reaction coordinate of ET reactions are identified that involve the reorganization of the hydrogen bond network and structural relaxation of the active site. The simulations reveal complex active site relaxation dynamics involving distinguished amino acids (Lys246, His365, and His369), conformational reorganization of the hydroxyl group of the (6-4) photoproduct, and a strengthening of hydrogen bonds with immobilized water molecules. In particular, rotation of the Lys246 side chain is found to impose a double-well character along the reaction coordinate of the ET reaction. Our findings suggest that the primary ET reactions in the (6-4) photoproduct enzyme repair complex of D. melanogaster are governed by a complex multi-minima active site relaxation dynamics and potentially precede the equilibration of the protein. ET pathways mediated by the adenine moiety and the 5' side of the photoproduct are proposed to be relevant for triggering the catalytic (6-4) photoproduct reactivation.

A Class II CPD Photolyase and a 6-4 Photolyase with Photorepair Activity from the Antarctic Moss Pohlia nutans M211

Antarctic mosses are the dominant vegetation in the Antarctic continent. Because of stratospheric ozone depletion, they can withstand physiological extreme UV. The formation of CPD and 6-4PP is one of the most harmful damages of UV to DNA. DNA damage can interfere with replication and transcription, resulting in mutation and death. Two types of photolyase, CPD photolyase and 6-4 photolyase, are capable of specific binding CPD or 6-4PP and repairing these lesions. However, there is little research on photolyase in Antarctic moss. Here, we isolated a gene encoding class II CPD photolyase (PnCPDPhr) and a gene encoding 6-4 photolyase (Pn6-4Phr) from Antarctic moss P. nutans M211. When exposed to UVB, CPDs accumulated in gametophytes and the gene expressions of PnCPDPhr and Pn6-4Phr were both up-regulated. In addition, the in vitro expression and photoreactivation assays of PnCPDPhr and Pn6-4Phr were performed. Our results demonstrated that PnCPDPhr and Pn6-4Phr have an effective activity of DNA repair. This is the first study to determine the CPD accumulation in Antarctic moss as well as the first report isolating CPD photolyase and 6-4 photolyase from Antarctic moss. These results will enrich the knowledge of photolyase family and benefit the exploitation of functioning gene in Antarctic moss.

Theoretical insights into the DNA repair function of Arabidopsis thaliana cryptochrome-DASH

Following the discovery of cryptochrome-DASH (CRYD) as a new type of blue-light receptor cryptochrome, theoretical and experimental findings on CRYD have been reported. Early studies identified CRYD as highly homologous to the DNA repair enzyme photolyases (PLs), suggesting the involvement of CRYD in DNA repair. However, an experimental study reported that CRYD does not exhibit DNA repair activity in vivo. Successful PL-mediated DNA repair requires: (i) the recognition of UV-induced DNA lesions and (ii) an electron transfer reaction. If either of them is inefficient, the DNA repair activity will be low.

To elucidate the functional differences between CRYD and PL, we theoretically investigated the electron transfer reactivity and DNA binding affinity of CRYD and also performed supplementary experiments. The average electronic coupling matrix elements value for Arabidopsis thaliana CRYD (AtCRYD) was estimated to be 5.3 meV, comparable to that of Anacystis nidulans cyclobutane pyrimidine dimer PLs (AnPL) at 4.5 meV, indicating similar electron transfer reactivities. We also confirmed the DNA repair activity of AtCRYD for UV-damaged single-stranded DNA by the experimental analysis. In addition, we investigated the dynamic behavior of AtCRYD and AnPL in complex with double-stranded DNA using molecular dynamics simulations and observed the formation of a transient salt bridge between protein and DNA in AtCRYD, in contrast to AnPL in which it was formed stably. We suggested that the instability of the salt bridge between protein and DNA will lead to reduced DNA binding affinity for AtCRYD.

Formation and Recognition of UV-Induced DNA Damage within Genome Complexity

Ultraviolet (UV) light is a natural genotoxic agent leading to the formation of photolesions endangering the genomic integrity and thereby the survival of living organisms. To prevent the mutagenetic effect of UV, several specific DNA repair mechanisms are mobilized to accurately maintain genome integrity at photodamaged sites within the complexity of genome structures. However, a fundamental gap remains to be filled in the identification and characterization of factors at the nexus of UV-induced DNA damage, DNA repair, and epigenetics. This review brings together the impact of the epigenomic context on the susceptibility of genomic regions to form photodamage and focuses on the mechanisms of photolesions recognition through the different DNA repair pathways.