NONCOVALENT DRUG-DNA BINDING INTERACTIONS THAT INHIBIT AND STIMULATE (A)BC EXCINUCLEASE

(A)BC excinuclease from Escherichia coli catalyzes the initial step of nucleotide excision repair. It recognizes and binds to many types of covalent modifications in DNA and incises the damaged strand on both sides of the lesion. We employed a variety of noncovalent DNA binding drugs to examine in vitro the mechanisms and the nature of the DNA-drug interactions responsible for two phenomena: inhibition of excision repair by caffeine and other noncovalent DNA binding compounds; incision of undamaged DNA produced by (A)BC excinuclease in the presence of the bisintercalating drug ditercalinium. All of the chemicals examined (e.g., actinomycin D, caffeine, ethidium bromide, and Hoechst 33258) inhibited incision of a covalent adduct by (A)BC excinuclease, and direct evidence is given for a common mechanism in which UvrA is depleted by binding to drug-undamaged DNA complexes. In the absence of significant amounts of undamaged DNA, another mechanism of inhibition was observed, in which enzyme bound to noncovalent drug-DNA complexes in the vicinity of the lesion prevents formation of preincision complexes at the lesion. Ditercalinium and unexpectedly all of the other drugs examined promoted the incision of undamaged DNA when the enzyme was present at high concentration. Thus, this activity contrary to previous assumptions is not unique to bisintercalators. Another unexpected finding was stimulation of incision at certain sites of photodamage in DNA produced by low concentrations of noncovalent DNA binding chemicals. Thus, enzyme inhibition, the stimulation of incision at certain damaged sites, and the production of enzymatic incisions at undamaged sites in DNA are effects that many noncovalent DNA binding chemicals have on (A)BC excinuclease, including intercalating drugs and minor groove binders with high and low sequence specificity.