Direct repair

In addition to proofreading and the excision repair mechanisms, the Bacterial cell has a range of mechanisms for correcting chemical changes in DNA in situ. The DNA photo- lyases are used for photoreactivation or light repair of pyrimidine dimers and proteins such as O6-methylguanine-DNA methyltransferase the Ada protein can repair the effects of alkylating agents such as MMS methylmethane sulfonate. This transferase is unusual in that it is not an enzyme in the strictest sense of the word. The methyl group is directly transferred to the Ada protein which renders it completely inactive. Thus it can- not participate in another reaction, so does not play a catalytic role.