1 H NMR Studies of the Binding of Bacteriophage‐M13‐Encoded Gene‐5 Protein to Oligo(deoxyadeny1ic acid)s of Varying Length

The binding of gene‐5 protein to oligo(deoxyadeny1ic acid)s varying in length from 2 to 16 nucleotides has been studied by titrating the protein with the oligonucleotides and recording the 1 H NMR spectra at 360 MHz. To obtain information about the mode of binding of the protein the aromatic parts of the spectra have been analysed by performing spectral simulations, starting from the assignments obtained from nuclear Overhauser enhancements at 500 MHz [Alma, N. C. M., Harmsen, B. J. M., Hull, W. E., Van der Marel, G., Van Boom, J. H., and Hilbers, C. W. (1981) Biochemisrry , 20, 4419‐4428]. The 1 H NMR spectra of the complexes of gene‐5 protein with (dA) 8 , (dA) 12 and (dA) 16 appear to be identical except for differences in linewidth. The 1 H NMR spectra of the complexes with the smaller oligonucleotides (dA) 2 , (dA) 3 and (dA) 4 differ from each other and from the spectra obtained from the complexes with longer oligonucleotides. However, binding of all oligo‐ nucleotides basically influences the same aromatic residues, namely two tyrosines and one phenylalanine. In the protein‐oligonucleotide complexes, one protein monomer covers three nucleotide residues, in contrast to the stoichiometry of 1: 4 found for protein‐polynucleotide complexes. It was found that the binding to oligo‐ nucleotides is cooperative and ionic‐strength‐dependent but far less so than found for the binding to poly‐ nucleotides.