The invention is in the field of regulation of enzymatic activity in nucleic acid modifying reactions. It describes a method of regulating enzymatic activity by adding chelating agents to the reaction composition and exploits the fact that both the binding of divalent cations to these chelating agents and the pH of commonly used buffers is temperature dependent. PCR experiments that are hampered by non-specific side products can be regulated such that the target sequence is amplified in a more specific manner.