Coliphage 434 tofprotein: NH2-terminal amino acid sequence and kinetic and equilibrium measurements of DNA binding

Summary Coliphage 434 tof protein was purified to a substantially pure state from imm⁴³⁴cI dv carrier cells. The minimum molecular weight is 7,500±500 as estimated by polyacrylamide gel electrophoresis. The amino acid sequence of the nine NH₂-terminal residues was determined, by manual Edman degradation of the intact protein, to be Met-Gln-Thr-Leu-Ser-Glu-Arg-Leu-(Lys)-.The purified protein at low concentrations binds specifically to imm⁴³⁴dv DNA and at high concentrations also binds to imm²¹dv and dv DNA. The curve of the specific binding is of Michaelis type, while that of the nonspecific binding is sigmoidal. The specific binding does not show marked temperature dependency at 4°–37°C. We have analyzed the equilibrium and kinetic data of specific binding. The equilibrium dissociation constant is 1.9x10^-11M at 0°C. The association rate constant and the dissociation rate constant are 1.1–2.9x10⁸M^-1s^-1 and 2.7x10^-3s^-1, respectively, at 0°C. The half life of the tof protein-operator DNA complex is 260s. These results suggest that the tof protein-operator interaction is much weaker than the interaction between the cI repressor and the operator reported by other workers.