Abstract
DNA-protein cross-links (DPCs) are produced by ionizing radiation, ultraviolet, aldehydes, and chemotherapeutic platinum compounds. DPCs would inhibit DNA replication and transcription by the steric hindrance of cross-linked proteins and hence exert adverse effects on cells. However, the repair mechanism of DPCs remains largely unknown. In the present study, we have examined the sensitivity of Escherichia coli cells to DPC-inducing agents to elucidate the repair mechanism of DPCs. To dissect repair pathways, Escherichia coli strains deficient in nucleotide excision repair (uvrA), recombinational repair (recA), and translesion synthesis (umuDC) were used. Cells were treated with formaldehyde or 5-azacytidine, and survival was measured by colony formation. It is known that formaldehyde produces a covalent linkage between amino groups of a DNA base and protein and that 5-azacytidine is metabolized and incorporated into DNA to produce a covalent linkage between 5-azacytosine and deoxycytidine methyltransferase. When cells were treated with formaldehyde, the survival fractions of uvrA and recA mutants but not a umuDC mutant were significantly lower than that of the wild type. In contrast, a recA mutant but not uvrA and umuDC mutants exhibited sensitivity to 5-azacytidine. These results suggest that DPCs formed by formaldehyde are repaired by both nucleotide excision and recombinational repair pathways, whereas DPCs formed by 5-azacytidine are repaired predominantly by the recombinational repair pathway.
