Ada, also called as O6 alkyl guanine transferase I (O6 AGT I), is an enzyme induced by treatment of bacterial cells with alkylating agents that mainly cause methylation damage.
Ada transfers the alkyl group from DNA bases and sugar-phosphate backbone to a cysteine residue, inactivating itself.
Consequently, it reacts stoichiometrically with its substrate rather than catalytically and is referred to as a suicide enzyme.
Methylation of Ada protein converts it into a self transcriptional activator, inducing its own gene expression and the expression of other genes which together with Ada help the cells repair alkylation damage.
[1] Ada removes the alkyl group attached to DNA bases like guanine (O6-alkyl guanine) or thymine (O4-alkyl thymine) and to the oxygen of the phosphodiester backbone of the DNA.