The enfolding arms of EcoRI endonuclease: role in DNA binding and cleavage.

The N-terminal segments of the EcoRI endonuclease dimer form part of mobile "arms" that encircle DNA in the recognition complex. By treating endonuclease-TCGCGAATTCGCG complexes with proteases, we have prepared a series of deletion derivatives lacking defined segments of the N-terminal region. The 5-12 segment is essential for DNA cleavage and forms one electrostatic interaction (per subunit) with DNA phosphate. These ionic contacts are directly across the double helix from the scissile phosphodiester bonds; they thus may permit the enfolding arms to immobilize DNA in apposition to the catalytic cleft and/or contribute to the unusual "kinked" conformation of DNA in the complex. Sequence specificity is fully retained when 28 residues are deleted from the N-terminus, but the complexes dissociate more rapidly.