| Abstract: | Alkylation interference and protection methods (Siebenlist, U., and Gilbert, W., (1980) Proc. Natl. Acad. Sci. U. S. A. 77, 122-126) have been utilized to deduce potential DNA contacts involved in specific complex formation between Eco RI endonuclease and its recognition sequence. The endonuclease protected the N7 position (major groove) of the dG and the N3 position (minor groove) of both dA residues within the Eco RI sequence against alkylation by dimethylsulfate, d(GpApApTpTpC), suggesting the presence of poly-peptide in both grooves in the vicinity of affected nitrogens. Results of methylation interference analysis suggest that the N7 of the Eco RI site dG and the N3 of the central dA, d(GpApApTpTpC), are utilized as contacts by the enzyme. The failure to observe interference upon methylation of the 5'-penultimate dA within the sequence implies that the endonuclease does not bond to the N3 of this residue, despite the fact that it is protected against alkylation by the protein. Ethylation interference patterns suggest four major phosphate contacts between endonuclease and each DNA strand. Two of these phosphates are 5'-external to the Eco RI sequence, d(pNpGpApApTpTpC), suggesting involvement of outside phosphates in electrostatic interactions. Moreover, alkylation protection and interference effects on the two DNA strands display perfect 2-fold symmetry. Thus, the endonuclease interacts with a minimum of 10 nucleotide pairs to yield a DNA-protein complex characterized by elements of symmetry. In contrast, specific alkylation effects were not observed in comparable experiments with the endonuclease and a DNA which had been previously methylated by the Eco RI modification enzyme. |
