Kinetic Studies of the Interaction between MS2 Phage and F Pilus of Escherichia coli

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Abstract
The kinetics of the binding reaction of MS2 phage to free F pili, which were highly purified from E. coli, was studied using a membrane filter assay. The rate of dissociation (kd) of the MS2-phage-F-pilus complex is very slow and follows 1st-order kinetics with a half-life of 4.2 h at 30.degree. C in the standard buffer. The dissociation rate is rather insensitive to temperature, but becomes more rapid at high ionic strength or at basic pH. In a 0.25 M ionic strength buffer, the half-life of the complex is about 1.0 min. The rate of association is very fast and follows 2nd-order kinetics with the rate constant for association (ka) being 8 .times. 107 M-1 s-1 at 30.degree. C in the standard buffer. The ka is almost insensitive to ionic strength but slightly sensitive to pH or temperature. Monovalent cations can also promote the binding reaction and divalent cations but the complex formed with monovalent cation is unstable. A study of the kinetics of dissociation suggests that there are 2 types of interaction between MS2 phage and F pilus; one is a strong interaction formed with divalent cations and the other is a weak 1 formed with monovalent cations. The physical nature of the bonds involved in the former and the latter seems to be mainly electrostatic and non-electrostatic, respectively. The mechanism of the binding reaction is discussed.