EcoRII Restriction Endonuclease Forms Specific Contacts to the Bases of Its Target Sequence Flipped from DNA in a Transition Complex with Photoactivatable Substrates

Abstract

The photoactivatable modified oligonucleotides were used to investigate direct contacts formed by the type IIE EcoRII restriction endonuclease and the T/A bases of its recognition site (5'-CCT/AGG). EcoRII dimer consists of a central catalytic core, made of two C-terminal endonuclease-like domains (EcoRII-C) from different subunits, and two N-terminal effector DNA binding domains (EcoRII-N). According to co-crystal structure of isolated EcoRII-C with DNA catalytic dimer EcoRII-C flips nucleotides of the central T/A pair into the enzyme binding pockets. Нere, photocross-linking technique was used to investigate the direct contacts formed by extrahelical T/A bases in the protein pockets of full-length EcoRII within the pre-reactive EcoRII–DNA complex obtained in the presence of Ca2+ in solution. Photoreactive zero-length agent 5-iodo-2'-deoxyuridine (IdU) was introduced as single substituent into the central T/A position of EcoRII recognition site or into the flanking nucleotide sequences of 14-mer DNA substrate. The substitution of only dT or dA residues in EcoRII recognition site resulted in formation of photocross-links upon irradiation only in the presence of Ca2+. Proteolytic digestion of the enzyme-oligonucleotide conjugates followed by MALDI-MS analysis have allowed to identify the 224VEYD227 EcoRII region involved in the formation of the cross-links. This region belongs to the central part of H-10 α-helix. Y226 residue was suggested to form cross-link with T or A bases of EcoRII site replaced by IdU within the pre-reactive complex. The flipped base pair protein pockets of EcoRII seem to accommodate equally well both A and T bases of the DNA substrate. Altogether, IdU-containing photoactivatable DNA substrates have allowed to trap the flipped bases in complex with full-length EcoRII before DNA cleavage in the solution and to identify direct enzyme–DNA contacts important for high specificity of EcoRII for the Т/A nucleotides providing a highly specific cleavage reaction.