Alpha-deoxyadenosine, a major anoxic radiolysis product of adenine in DNA, is a substrate for Escherichia coli endonuclease IV.

Oligonucleotides containing a unique alpha-deoxyadenosine or tetrahydrofuran (a model abasic site) were synthesized using phosphoramidite chemistry. Repair enzymes from Escherichia coli, including endonucleases III, IV, and VIII, exonuclease III, formamidopyrimidine N-glycosylase, and deoxyinosine 3'-endonuclease, as well as UV dimer N-glycosylases from T4 (den V) and Micrococcus luteus, were examined for their ability to recognize alpha-deoxyadenosine and tetrahydrofuran. In agreement with prior studies, a tetrahydrofuran-containing oligonucleotide was a substrate for endonuclease IV and exonuclease III, but not for the other repair enzymes. However, an oligonucleotide containing alpha-deoxyadenine was a substrate only for endonuclease IV. Competitive inhibition studies with both substrates confirmed that the activity recognizing alpha-deoxyadenine was endonuclease IV and not a possible contaminant in the endonuclease IV preparation. Using E. coli extracts, the activity that recognized alpha-deoxyadenine was dependent on nfo, the structural gene of endonuclease IV, further substantiating that endonuclease IV is the enzyme that recognized alpha-deoxyadenine. Kinetic measurements indicated that alpha-deoxyadenosine was as good a substrate for endonuclease IV as tetrahydrofuran; the Km and Vmax values for both substrates were similar. Using substrates that were labeled at either the 3'- or 5'-terminus, endonuclease IV was shown to hydrolyze the phosphodiester bond 5' to either alpha-deoxyadenosine or tetrahydrofuran, leaving the lesion, alpha-deoxyadenosine or tetrahydrofuran, on the 5'-terminus of the nicked site. The ability of endonuclease IV to recognize alpha-deoxyadenosine suggests that endonuclease IV is able to recognize a new class of DNA base lesions that is not recognized by other DNA N-glycosylases and AP endonucleases.