Interaction of endonuclease ecoRI with short specific and nonspecific oligonucleotides

The interaction of EcoRI with different oligodeoxyribonucleotides (ODNs) was analyzed using the method of the slow step-by-step simplification in their complexity. Orthophosphate (KI = 31 mM), 2-deoxyribose 5-phosphate (KI = 4.6 mM) and different dNMPs (KI = 2.1-2.5 mM) were shown to be the minimal ligands of the enzyme. The lengthening of a nonspecific d(pN)n (n = 1-6) by one nucleotide unit resulted in the increase of their affinity by a factor of approximately 2.0. Weak nonspecific electrostatic contacts of EcoRI with internucleotide phosphate groups of ODNs can account for about 5 orders of magnitude in the ligand affinity, whereas the contribution of specific interactions between EcoRI and d(pN)n is no more than 2 orders of magnitude of a total ODN's affinity.