SOLUTION STRUCTURE OF THE DNA METHYL PHOSPHOTRIESTER REPAIR DOMAIN OFESCHERICHIA-COLI ADA

The Escherichia coli Ada protein repairs methyl phosphotriesters in DN A by direct, irreversible methyl transfer to one of its own cysteine r esidues. The methyl-transfer process appears to be autocatalyzed by co ordination of the acceptor residue, Cys-69, to a tightly bound zinc io n. Upon methyl transfer, Ada acquires the ability to bind DNA sequence -specifically and thereby to induce genes that confer resistance to me thylating agents. The solution structure of an N-terminal 10-kDa fragm ent of Ada, which retains zinc binding and DNA methyl phosphotriester repair activities, was determined using multidimensional heteronuclear nuclear magnetic resonance techniques. The structure reveals a zinc-b inding motif unlike any observed thus far in transcription factors or zinc-containing enzymes and provides insight into the mechanism of met alloactivated DNA repair.