REPAIR OF DNA METHYLPHOSPHOTRIESTERS THROUGH A METALLOACTIVATED CYSTEINE NUCLEOPHILE

The Escherichia coli Ada protein repairs methylphosphotriesters in DNA by direct, irreversible methyl transfer to one of its own cysteines. Upon methyl transfer, Ada acquires the ability to bind specific DNA se quences and thereby to induce genes that confer resistance to methylat ing agents. The amino-terminal domain of Ada, which comprises the meth ylphosphotriester repair and sequence-specific DNA binding elements, c ontains a tightly bound zinc ion. Analysis of the zinc binding site by cadmium-113 nuclear magnetic resonance and site-directed mutagenesis revealed that zinc participates in the autocatalytic activation of the active site cysteine and may also function as a conformational switch .