DNA "melting" proteins. IV. Fluorescence measurements of binding parameters for bacteriophage T4 gene 32-protein to mono-, oligo-, and polynucleotides.

The quenching of the intrinsic tryptophan fluorescence of T4-coded gene 32-protein on binding to nucleotide ligands, which was described in the preceding paper, is here exploited to measure thermodynamic parameters of the single-stranded nucleic acid-gene 32-protein interaction. It is shown that binding of small ligands follows a single site binding isotherm, with association constants increasing from approximately 20 M-1 for phosphate, to approximately 10(3) M for ribose or deoxyribose 5'-phosphate, to approximately 10(4) M-1 for mononucleotides, and to approximately 10(5) M-1 for dinucleoside monophosphates (all in 0.1 M Na+). The measured binding constants appear to be about the same for homologous ribose- and deoxyribose-containing ligands and to be independent of oligonucleotide base sequence and composition. Furthermore, beyond the dinucleotide level and up to octanucleotides, the increase in binding constant with increasing chain length is only about that expected from the statistical factor resulting from the increased number of ways a longer oligonucleotide can form a protein complex. This suggests that the basic binding unit involved in gene 32-protein associations with single-stranded nucleic acids can be approximated by a dinucleoside monophosphate. Oligonucleotides long enough to accomodate two or more protein monomers are characterized by much larger association constants, indicating that binding is cooperative in protein concentration. A cooperativity parameter (omegac) of approximately 10(3) is estimated from these data, in good agreement with that deduced from the application of ligand-perturbed helix in equilibrium coil transition calculations. Values of association constants (Kcomegac) of approximately 10(8) M-1 (in 0.1 M Na+) and site size (nc) of approximately 5 (+/-1) nucleotide residues/protein monomer are determined by the fluorescence titration technique for the cooperative binding of gene 32-protein to both poly(dA) and poly(rA); these values are also in agreement with those measured by Jensen et al. (Jensen, D.E. Kelly, R.C., and von Hippel, P.H. (1976) J. Biol. Chem. 251, 7215-7228). Possible in vivo consequences and correlations of these findings with proposed roles for gene 32-protein in replication and recombination are discussed.