[Interaction of EcoRII restriction and modification enzymes with synthetic DNA fragments. X. Hydrolysis of substrates with structural abnormalities].

Interaction of the EcoRII restriction endonuclease with a set of 30-membered substrates having structural anomalies in the recognition site (decreases CCT/AGG) and in adjacent sequences has been studied. A nick in the centre of the EcoRII recognition site between dC and dA residues slows down hydrolysis of the nonmodified strand, whereas the modified one is not cleaved. Removal of the phosphate group from the nick in this substrate does not alter the rate of the cleavage. The absence of one of the phosphate groups in the flanking sequence at a two-base-pair "distance" from the recognition site slows down the enzymatic hydrolysis. Removal of dA or dT out of the EcoRII recognition site blocks the enzymatic reaction. It appears that EcoRII does not interact with the phosphate group between dC and dA residues in the recognition site. Suggestions are made concerning possible contacts of the EcoRII restriction endonuclease with dA- and dT-residues of the recognition site and with the sugar-phosphate backbone of the adjacent nucleotide sequences.