Kinetics of the E. coli replication factor DnaC protein-nucleotide interactions. II. Fluorescence anisotropy and transient, dynamic quenching stopped-flow studies of the reaction intermediates.

The nature of the intermediates in the binding of MANT-ATP and MANT-ADP to the E. coli replicative factor DnaC protein (accompanying paper) has been examined using the fluorescence intensity, anisotropy, and transient dynamic quenching stopped-flow techniques. Using molar fluorescence intensities of individual intermediates of the reaction, we derived the Stern-Volmer equation that provides a direct method to quantitatively address the quenching of the fluorescence of a transient intermediate by an external, neutral quencher. The data indicate that in the first intermediate, (C)(1), the solvent has full access to the MANT group. Thus, the nucleotide-binding site is located on the surface of the protein, fully open to the solvent. Moreover, formation of the first intermediate does not affect the structure of the binding site. On the other hand, in the second intermediate, (C)(2), the entire binding site changes its conformation, resulting in diminished access of the solvent to the bound nucleotide. The time course of the fluorescence anisotropy in the reaction provides direct, unique insight into the mobility of bound nucleotides in each intermediate. The analysis is facilitated by the fact that the anisotropy can be expressed as a function of the relative molar intensities and steady-state anisotropies of the individual intermediates. The major decrease of the nucleotide mobility occurs in the formation of the first intermediate and reflects the fact that the MANT group is immobilized to a similar extent as the ribose region of the bound nucleotides. Transition to the second intermediate and closing of the binding site leads to only a moderate, additional decrease of nucleotide mobility. The temperature effect on the studied interactions indicates that the formation of individual intermediates is accompanied by very different enthalpy and entropy changes predominantly generated from the structural changes of the protein. Analysis of the salt effect indicates that the net release of a single ion, observed in equilibrium studies, occurs in the formation of the first intermediate. The lack of any salt effect on the (C)(1) <--> (C)(2) transition indicates that the closing of the binding site does not include a net ion release or uptake. Moreover, prior to the nucleotide binding, the conformational transition of the DnaC protein is exclusively controlled by the nucleotide binding and release.