Uncoupling of the recBC ATPase from DNase by DNA Crosslinked with Psoralen

Exonucleolytic cleavage of DNA by therecBC DNase is accompained by a DNA-dependent ATP hydrolysis that ceases when the DNA that has been digested to a limit. On the other hand, DNA that has been crosslinked by 4,5′,8-trimethylpsoralen in the presence of 360-nm light remains an effective cofactor in the ATPase reaction, but is resistant to digestion by the enzyme. Psoralentreated DNA is degraded by pancreatic DNase, micrococcal nuclease, andEscherichia coli B restriction enzyme, but not byNeurospora crassa nuclease, suggesting that crosslinking did not grossly distort the duplex structure of the DNA. The psoralen-DNA is not a potent inhibitor, but competes with single-stranded DNA from bacteriophage fd for therecBC DNase to roughly the same extent as does normal duplex DNA. DNA treated with psoralen in the dark, exposed to 360-nm light in the absence of psoralen, or treated with the intercalating agents ethidium bromide, 9-aminoacridine, ICR-191, or actinomycin D, responds to the enzyme no differently from untreated DNA. However, DNA crosslinked with mitomycin C or nitrogen mustard behaves similarly to psoralen-treated DNA. The relationship of these findings to models for the function and control of therecBC ATPase and nuclease, and the advantages of psoralen as a DNA crosslinking agent, are discussed.