The formation, identification, and significance of DNA-protein cross-links in mammalian cells.

DNA-protein cross-links (DPC) are formed by a variety of radiations and chemicals which act via free radical formation. Covalency is inferred from the resistance of the cross-links to harsh treatments. In mammalian cells, a background of DPC (6000 per V79 cell) may result from normal associations of chromosomal loops with the nuclear protein matrix. After ionizing radiation, the elevated level of DPC (150 per Gy per V79 cell) are enriched in actively transcribing DNA and in a subset of proteins of the nuclear matrix. DPC formation is reduced by hydroxyl radical scavengers, by oxygen, and by hypertonic medium and is enhanced by hypotonic medium and by removal of intracellular glutathione. DPC are repaired more slowly than single-strand breaks and not at all when formed during metaphase. During the postirradiation period, changes in the sequence composition of the DNA of residual DPC are consistent with the preferential repair of DPC in actively expressed genes. Excision repair mechanisms have been proposed. Unrepaired DPC may block normal functions of the nuclear matrix, such as replication and transcription.