DNA-protein crosslinks are relatively common DNA lesions that form during the physiological processing of DNA by replication and recombination proteins, by side reactions of base excision repair enzymes, and by cellular exposure to bifunctional DNA-damaging agents such as platinum compounds. The mechanism by which pathological DNA-protein crosslinks are repaired in humans is not known. In this study, we investigated the mechanism of recognition and repair of protein-DNA and oligopeptide-DNA crosslinks by the human excision nuclease. Under our assay conditions, the human nucleotide excision repair system did not remove a 16-kDa protein crosslinked to DNA at a detectable level. However, 4-and 12-aa-long oligopeptides crosslinked to the DNA backbone were recognized by some of the damage recognition factors of the human excision nuclease with moderate selectivity and were excised from DNA at relatively efficient rates. Our data suggest that, if coupled with proteolytic degradation of the crosslinked protein, the human excision nuclease may be the major enzyme system for eliminating protein-DNA crosslinks from the genome.damage recognition ͉ nucleotide excision repair D NA-protein crosslinks are intermediates in the reaction pathways of certain enzymes such as integrases, topoisomerases, and meiotic recombinases (1-4). In addition, crosslinks are induced by DNA-damaging agents including ionizing radiation, UV light, metals such as chromium and arsenic (5, 6), and bifunctional chemotherapeutic drugs such as platinum compounds and nitrogen mustards (7-9). Finally, crosslinks may form when repair enzymes attempt to process aldehyde groups either in the DNA backbone in the form of apurinic͞ apyrimidinic (AP) sites or on oxidized nucleobase residues (10).The repair of some of these DNA-protein crosslinks is well understood: the topoisomerase I-DNA 3Ј tyrosyl-phosphodiester is commonly cleaved in the normal course of the topoisomerase (topo) reaction. However, when topo cleaves DNA near lesions such as pyrimidine dimers or nicks, the enzyme-DNA crosslink is frozen in transit, giving rise to a stable crosslink at a single-stranded nick site. This lesion is repaired by a remarkable enzyme called tyrosyl-DNA phosphodiesterase, Tdp1 (1). Tdp1 cleaves the 3Ј tyrosyl-phosphodiester bond and converts it to a 3Ј phosphate terminus, which is then processed to a 3Ј OH that can be ligated. Tdp1 has also been implicated in the removal of other 3Ј adducts (2). The enzyme has been found in all eukaryotes tested in which a topo I-3Ј tyrosylphosphodiester bond forms in the course of the reaction but not in prokaryotes in which the reaction intermediate is a topo I-DNA 5Ј tyrosyl-phosphodiester (3).DNA-protein crosslinks can also form during meiosis. Spo11 in yeast (SPO11 in mammals) makes a staggered double-strand break to initiate meiotic recombination; each subunit of the dimeric protein is linked to the resulting 5Ј ends through a 5Ј tyrosyl-phosphodiester linkage. In contrast to the topo I-3Ј tyrosyl-phosphodiester complex, the Spo11-...