O⁶-Benzylguanine Stimulates Regulatory Functions of the Ada Protein in Escherichia coli

Abstract

In vitro experiments showed that O⁶-benzylguanine (O⁶-benzG, 0.2 µM) fully inhibited the repair activity of human O⁶-alkylguanine-DNA alkyltransferase (MGMT) due to the formation of S-benzylcysteine in the protein acceptor site. O⁶-benzG at concentrations increased many times (up to 800 µM) failed to inhibit the repair activity of the Escherichia coli Ada protein, the structural and functional analog of MGMT. It has been shown for the first time that O⁶-benzG stimulates the regulatory activity of the Ada protein. In experiments with N-nitroso-N-methylurea (MNU), the pretreatment of Escherichia coli cells with O⁶-benzG at a sublethal concentration of 10 µM led to a twofold enhancement of transcription at the Ada-dependent promoter of the alkA gene in control cells and ensured transcription enhanced 1.6–1.7 times at alkA and alkB promoters in cells with the induced “classical” Ada response. Apparently, an increase in the regulatory activity of the Ada protein was associated with the formation of the stable protein molecule having the strong affinity for alkA and alkB promoters after transfer of the benzyl group from O⁶-benzG to the acceptor site Cys-69 in the N-terminal domain of Ada protein. O⁶-benzG did not affect the regulative activity of Ada in alternative quasi-adaptive responses to MNU.