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

Chromatin dynamics at DNA replication, transcription and repair

During DNA replication, transcription and DNA repair in eukaryotes, the cellular machineries performing these tasks need to gain access to the DNA that is packaged into chromatin in the nucleus. Chromatin is a dynamic structure that modulates the access of regulatory factors to the genetic material. A precise coordination and organization of events in opening and closing of the chromatin is crucial to ensure that the correct spatial and temporal epigenetic code is maintained within the eukaryotic genome. This review will summarize the current knowledge of how chromatin remodeling and histone modifying complexes cooperate to break and remake chromatin during nuclear processes on the DNA template.

Chromatin remodeling and the maintenance of genome integrity

DNA damage of any type is threatening for a cell. If lesions are left unrepaired, genomic instability can arise, faithful transmission of genetic information is greatly compromised eventually leading the cell to undergo apoptosis or carcinogenesis. In order to access/detect and repair these damages, repair factors must circumvent the natural repressive barrier of chromatin. This review will present recent progress showing the intricate link between chromatin, its remodeling and the DNA repair process. Several studies demonstrated that one of the first events following specific types of DNA damage is the phosphorylation of histone H2A. This mark or the damage itself are responsible for the association of chromatin-modifying complexes near damaged DNA. These complexes are able to change the chromatin structure around the wounded DNA in order to allow the repair machinery to gain access and repair the lesion. Chromatin modifiers include ATP-dependent remodelers such as SWI/SNF and Rad54 as well as histone acetyltransferases (HATs) like SAGA/NuA4-related complexes and p300/CBP, which have been shown to facilitate DNA accessibility and repair in different pathways leading to the maintenance of genome integrity.

Linking lipids to chromatin

Dynamic regulation of chromatin structure is thought to be a prerequisite for nuclear functions that require accessibility to DNA such as replication, transcription and DNA repair. The phosphoinositide (PI) pathway is a second messenger signalling system regulated in response to a variety of extracellular (growth factors, differentiation signals) and intracellular (cell cycle progression, DNA damage) stimuli. The presence of a PI pathway in the nucleus together with the recent findings that specific nuclear proteins can interact with and are regulated by phosphoinositides suggest that changes in the nuclear phosphoinositide profile may have a direct role in modulating chromatin structure.

Binding of zinc finger protein transcription factor IIIA to its cognate DNA sequence with single UV photoproducts at specific sites and its effect on DNA repair

The relationship between DNA repair efficiency at specific locations in the binding site of the nine-zinc finger protein transcription factor IIIA (TFIIIA) and binding of its individual zinc fingers was studied. Homogeneously damaged oligonucleotides, which contained a single cis-syn cyclobutane thymine dimer (CTD) at one of six different sites in the internal control region (ICR) of the 5 S rRNA gene to generate a series of damaged DNA substrates, were prepared by chemical synthesis. Binding of TFIIIA to the substrates was assayed by measurement of dissociation constants (Kd), dissociation rates (koff), and protein-DNA contacts. The results indicated that a single CTD in the ICR does not significantly affect the Kd of TFIIIA. In contrast, CTDs at positions +55 and +72 (from the transcription start site) in the ICR markedly enhanced koff of TFIIIA from the complex. In addition, CTDs in these two sites increased methylation of the N7 of guanines (by dimethyl sulfate) in the zinc finger contacts of the ICR-TFIIIA complex. Furthermore CTDs at +55 and +72 were more efficiently removed from the complex than CTDs at other sites in the ICR by Xenopus oocyte nuclear extracts. This suggests that repair of CTDs closely correlates with changes in the binding of individual zinc fingers of the ICR-TFIIIA complex. These results have implications for the mechanism of DNA damage recognition and repair in protein-DNA complexes.

Repair of UV lesions in silenced chromatin provides in vivo evidence for a compact chromatin structure

Genes positioned close to telomeres in yeast are silenced by a heterochromatin-like structure containing Sir proteins. To investigate whether silencing also affects DNA repair, we studied removal of UV lesions by photolyase and nucleotide excision repair (NER) in strains containing the URA3 gene inserted 2 kilobases from a telomere. URA3 was transcriptionally active in sir3delta mutants, partially silenced in SIR3 cells, or completely silenced by overexpression of SIR3 or deletion of RPD3. The active URA3 showed efficient repair by both pathways. Fast repair of the promoter and 3' end by photolyase reflected a non-nucleosomal structure. Partial silencing had no remarkable effect on photolyase but reduced repair by NER, indicating differential accessibility for the two repair reactions. Complete silencing inhibits NER and photolyase in the coding region as well as in the promoter and the 3'-end. Conventional nuclease footprinting analyses revealed subtle changes in the promoter proximal nucleosome under partially silenced conditions but a pronounced reorganization of chromatin extending over the whole gene in silenced chromatin. Thus, both repair systems are sensitive to chromatin changes associated with silencing and provide direct evidence for a compact structure of heterochromatin.