The effects of ultraviolet C on in vitro nucleosome assembly and stability

A half century of exploring DNA excision repair in chromatin

DNA in eukaryotic cells is packaged into the compact and dynamic structure of chromatin. This packaging is a double-edged sword for DNA repair and genomic stability. Chromatin restricts the access of repair proteins to DNA lesions embedded in nucleosomes and higher order chromatin structures. However, chromatin also serves as a signaling platform in which post-translational modifications of histones and other chromatin-bound proteins promote lesion recognition and repair. Similarly, chromatin modulates the formation of DNA damage, promoting or suppressing lesion formation depending on the chromatin context. Therefore, the modulation of DNA damage and its repair in chromatin is crucial to our understanding of the fate of potentially mutagenic and carcinogenic lesions in DNA. Here, we survey many of the landmark findings on DNA damage and repair in chromatin over the last 50 years (i.e., since the beginning of this field), focusing on excision repair, the first repair mechanism studied in the chromatin landscape. For example, we highlight how the impact of chromatin on these processes explains the distinct patterns of somatic mutations observed in cancer genomes.

In vitro chromatin templates to study nucleotide excision repair

In eukaryotic cells, DNA associates with histones and exists in the form of a chromatin hierarchy. Thus, it is generally believed that many eukaryotic cellular DNA processing events such as replication, transcription, recombination and DNA repair are influenced by the packaging of DNA into chromatin. This mini-review covers the current knowledge of DNA damage and repair in chromatin based on in vitro studies. Specifically, nucleosome assembly affects DNA damage formation in both random sequences and sequences with strong nucleosome-positioning signals such as 5S rDNA. At least three systems have been used to analyze the effect of nucleosome folding on nucleotide excision repair (NER) in vitro: (a) human cell extracts that have to rely on labeling of repair synthesis to monitor DNA repair, due to very low repair efficacy; (b) Xenopus oocyte nuclear extracts, that have very robust DNA repair efficacy, have been utilized to follow direct removal of DNA damage; (c) six purified human DNA repair factors (RPA, XPA, XPC, TFIIH, XPG, and XPF-ERCC1) that have been used to reconstitute excision repair in vitro. In general, the results have shown that nucleosome folding inhibits NER and, therefore, its activity must be enhanced by chromatin remodeling factors like SWI/SNF. In addition, binding of transcription factors such as TFIIIA to the 5S rDNA promoter also modulates NER efficacy.

Chromatin remodeling activities act on UV-damaged nucleosomes and modulate DNA damage accessibility to photolyase

Nucleosomes inhibit DNA repair in vitro, suggesting that chromatin remodeling activities might be required for efficient repair in vivo. To investigate how structural and dynamic properties of nucleosomes affect damage recognition and processing, we investigated repair of UV lesions by photolyase on a nucleosome positioned at one end of a 226-bp-long DNA fragment. Repair was slow in the nucleosome but efficient outside. No disruption or movement of the nucleosome was observed after UV irradiation and during repair. However, incubation with the nucleosome remodeling complex SWI/SNF and ATP altered the conformation of nucleosomal DNA as judged by UV photo-footprinting and promoted more homogeneous repair. Incubation with yISW2 and ATP moved the nucleosome to a more central position, thereby altering the repair pattern. This is the first demonstration that two different chromatin remodeling complexes can act on UV-damaged nucleosomes and modulate repair. Similar activities might relieve the inhibitory effect of nucleosomes on DNA repair processes in living cells.

DNA-repair by photolyase reveals dynamic properties of nucleosome positioning in vivo

Nucleosomes exert a repressive influence on the biological functions of DNA by restricting the access of proteins to DNA. To investigate how intrinsic properties of nucleosomes modulate DNA-accessibility in vivo, we studied DNA repair by photolyase in the yeast URA3 gene. Formation of DNA lesions (cyclobutane pyrimidine dimers, CPDs) and photolyase activity are controlled precisely by light. Preceding work revealed that photolyase repairs nucleosome-free DNA rapidly, while repair of nucleosomes is inhibited severely. The high-resolution data presented here show slow repair in the center of nucleosomes and a gradual increase towards the periphery. This pattern was observed in all nucleosomes and demonstrates that dynamic properties facilitate DNA accessibility. Since the URA3 nucleosomes can occupy alternate positions, the repair data are most consistent with nucleosome mobility that moves CPDs in linker DNA where they are repaired rapidly. A partial and transient unfolding or disruption of nucleosomes, however, may not be excluded. In addition, repair heterogeneity was found between closely spaced sites, indicating that structural properties of nucleosomes contribute to damage processing. Moreover, nucleosome-specific modulation of photolyase was found on the transcribed and non-transcribed strand. This is in contrast to homogeneous repair of the transcribed strand by nucleotide excision repair, and reveals fundamental differences in how both repair systems interact with nucleosomes and transcription.

Photoreactivation of UV-induced cyclobutane pyrimidine dimers in the MFA2 gene of Saccharomyces cerevisiae

Photoreactive repair (PR) of cyclobutane pyrimidine dimers (CPDs) was mapped at nucleotide resolution in nucleotide excision repair (NER) proficient and deficient strains for the transcriptionally active and inactive MFA2 gene. Enhanced PR in the control region occurred in areas where no nucleosomes were present, particularly linker regions in the alpha mating type. The presence of excision plus transcriptional activation alleviated this preference, causing repair in the regions that were linker and core in the alpha mating type to be the same in this strain. Transcription had no effect on photoreactive repair in transcribed and downstream regions of MFA2, where similar rates were observed for specific CPDs in both strands. The presence of positioned nucleosomes in alpha mating types revealed slow repair in the nucleosome core, with faster repair occurring at the 3' and 5' edges. These data support the evidence that CPDs are repaired quicker in nucleosome-free regions and at edges of nucleosomes. CPDs in the linker regions are repaired more efficiently in the transcriptionally inactive strains, suggesting that nucleosome movement associated with transcription of MFA2 hampers PR irrespective of the strand. Proficient NER influenced PR in the TATA and Mcm1 binding sites by enhancing it, particularly when transcription was activated.

Modulation of DNA damage and DNA repair in chromatin

DNA is packaged in the highly compact and dynamic structure of chromatin in eukaryotic cells. It is generally accepted that DNA processing events in the nucleus, such as transcription, replication, recombination, and repair, are restricted by this packaging. For some processes (e.g., transcription), the chromatin fiber is "preset" in a more open structure to allow access of proteins to specific regions of DNA within this structural hierarchy. These regions contain modified nucleosomes that accommodate a less compact state of chromatin and allow access to specific regions of DNA. DNA repair proteins, however, must access DNA lesions in all structural domains of chromatin after sudden insult to the genome. Damaged DNA must be recognized, removed, and replaced by repair enzymes at all levels of chromatin packaging. Therefore, the modulation of DNA damage and its repair in chromatin is crucial to our understanding of the fate of potential mutagenic and carcinogenic lesions in DNA. In this review, we discuss the modulation of DNA damage and DNA repair by chromatin structure, and the modulation of chromatin structure by these events.

DNA damage can alter the stability of nucleosomes: effects are dependent on damage type

We have investigated the effects of DNA damage by (+/-)-anti-benzo[a]pyrene diol epoxide (BPDE) and UV light on the formation of a positioned nucleosome in the Xenopus borealis 5S rRNA gene. Gel-shift analysis of the reconstituted products indicates that BPDE damage facilitates the formation of a nucleosome onto this sequence. Competitive reconstitution experiments show that average levels of 0.5, 0.9, and 2.1 BPDE adducts/146 bp of 5S DNA (i.e., the size of DNA associated with a nucleosome core particle) yield changes of -220, -290, and -540 cal/mol, respectively, in the free energy (delta G) of nucleosome formation. These values yield increases of core histone binding to 5S DNA (K(a)) of 1.4-, 1.6-, and 2.5-fold, compared with undamaged DNA. Conversely, irradiation with UV light decreases nucleosome formation. Irradiation at either 500 or 2500 J/m2 of UV light [0.6 and 0.8 cyclobutane pyrimidine dimer/146 bp (on average), respectively] results in respective changes of +130 and +250 cal/mol. This translates to decreases in core histone binding to irradiated 5S DNA (K(a)) of 1.2- and 1.5-fold compared with undamaged DNA. These results indicate that nucleosome stability can be markedly affected by the formation of certain DNA lesions. Such changes could have major effects on the kinetics of DNA processing events.

Preferential inhibition of nucleosome assembly by ultraviolet-induced (6-4)photoproducts

We reconstituted nucleosomes in vitro using two kinds of damaged pBR322 plasmid DNA carrying cyclobutane pyrimidine dimers (CPD) or (6-4)photoproducts. The results indicate that nucleosome assembly is inhibited preferentially by (6-4)photoproducts compared with CPD, suggesting that the regions carrying (6-4)photoproducts retain their nucleosome-free form, i.e. linker-like conformation until completion of the repair processes.