We describe the results of a systematic study, using electron microscopy, of the effects of ionic strength on the morphology of chromatin and of H1-depleted chromatin. With increasing ionic strength, chromatin folds up progressively from a filament of nucleosomes at -1 mM monovalent salt through some intermediate higher-order helical structures (Thoma,

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Nucleotide excision repair (NER) and DNA repair by photolyase in the presence of light (photoreactivation) are the major pathways to remove UV-induced DNA lesions from the genome, thereby preventing mutagenesis and cell death. Photoreactivation was found in many prokaryotic and eukaryotic organisms, but not in mammals, while NER seems to be universally distributed. Since packaging of eukaryotic DNA in nucleosomes and higher order chromatin structures affects DNA structure and accessibility, damage formation and repair are coupled intimately to structural and dynamic properties of chromatin. Here, I review recent progress in the study of repair of chromatin and transcribed genes. Photoreactivation and NER are discussed as examples of how an individual enzyme and a complex repair pathway, respectively, access DNA lesions in chromatin and how these two repair processes fulfil complementary roles in removal of UV lesions. These repair pathways provide insight into the structural and dynamic properties of chromatin and suggest how other DNA repair processes could work in chromatin. Keywords: chromatin/DNA repair/nucleotide excision repair/photolyase/transcription Chromatin, a dynamic and heterogeneous substrate for DNA repairChromatin is the packaged state of DNA in the nucleus and the substrate for all DNA-dependent processes, including transcription and DNA repair (Wolffe, 1995). At its lowest level of structural organization, DNA is folded in an array of nucleosomes called the nucleosome filament. The core of a nucleosome is composed of an octamer of four highly folded histone proteins, H2A, H2B, H3 and H4 (two of each), and 145 bp of DNA wrapped around the octamer. The nucleosome filament is formed by connecting nucleosome cores with linker DNA of variable length and by association of one histone H1 per nucleosome. The nucleosome filament is condensed further into compact 30 nm fibres and higher order structures. These structures are present in interphase and metaphase and are the target for damage formation and repair. There is pronounced structural and functional heterogeneity along chromosomal DNA with respect to DNA sequence, composition and functional activity, which projects into heterogeneity of damage formation and repair. This heterogeneity includes © European Molecular Biology Organization 6585 regions required for regulation of gene expression and replication, which associate with sequence-specific proteins and frequently are characterized by enhanced accessibility to nucleases (nuclease-sensitive regions; Workman and Kingston, 1998). Only a minor fraction of genes are transcribed or pre-set for transcription at any moment. The structure of these genes may include histone proteins modified by acetylation, altered or disrupted nucleosomes in the promoter regions, an unfolding of the whole chromatin domain, a loss or rearrangement of nucleosomes in the transcribed part, as well as differential loading of the genes with transcribing RNA polymerases. Moreover, a large fraction of genes are inactivated in h...

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