Nucleic Acids Research, 2002, Vol. 30, No. 18 3936-3944
© 2002 Oxford University Press
Interaction of the ocr gene 0.3 protein of bacteriophage T7 with EcoKI restriction/modification enzyme
Department of Chemistry, The King’s Buildings, University of Edinburgh, Edinburgh EH9 3JJ, UK
*To whom correspondence should be addressed. Tel: +44 131 650 4735; Fax: +44 131 650 6453; Email: david.dryden{at}ed.ac.uk
Present addresses:
C. Atanasiu, Wistar Institute, 3601 Spruce Street, Philadelphia, PA 19104, USA
S. S. Sturrock, Aneda Ltd, The Logan Building, Roslin BioCentre, Roslin, Midlothian EH25 9PS, UK
The ocr protein, the product of gene 0.3 of bacteriophage T7, is a structural mimic of the phosphate backbone of B-form DNA. In total it mimics 22 phosphate groups over 
24 bp of DNA. This mimicry allows it to block DNA binding by type I DNA restriction enzymes and to inhibit these enzymes. We have determined that multiple ocr dimers can bind stoichiometrically to the archetypal type I enzyme, EcoKI. One dimer binds to the core methyltransferase and two to the complete bifunctional restriction and modification enzyme. Ocr can also bind to the component subunits of EcoKI. Binding affinity to the methyltransferase core is extremely strong with a large favourable enthalpy change and an unfavourable entropy change. This strong interaction prevents the dissociation of the methyltransferase which occurs upon dilution of the enzyme. This stabilisation arises because the interaction appears to involve virtually the entire surface area of ocr and leads to the enzyme completely wrapping around ocr.
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