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Mechanism of Adsorption and Eclipse of Bacteriophage X174 III. Comparison of the Activation Parameters for the In Vitro and In Vivo Eclipse Reactions With Mutant and Wild-Type Virus

^a Department of Chemistry and Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida 32306

ABSTRACT

In a starvation buffer containing 10^–3 M divalent cations, X174 undergoes viral eclipse above 20 C when attached to intact host cells. An in vitro structural transition that is similar to that observed in this in vivo eclipse reaction occurs over the same temperature range in 0.1 M CaCl₂ (pH 7.2). Since both reactions result in a loss of infectivity, their kinetics have been compared in this report. Both exhibit a biphasic first-order loss in PFU that is a result of two competing first-order processes. However, a single type of heterogeneity in the population of virions is not the basis for both competing slower reactions. The Arrhenius plots of the faster components show that the in vitro eclipse reaction has the same activation energy of 35 kcal/mol (ca. 1.47 x 10⁵ J/mol) as the in vivo reaction but a 10-fold lower Arrhenius preexponential factor. This is further evidence that certain features of the in vivo mechanism are retained in the in vitro reaction. In the case of the slower components, the in vitro reaction has an activation energy of 37 kcal/mol (1.55 x 10⁵ J/mol), whereas that of the in vivo reaction is only 5 kcal/mol (2.1 x 10⁴ J/mol). A similar analysis has been performed on a cold-sensitive eclipse mutant of X174. In vivo, the mutation is expressed by a two- to three-fold lower Arrhenius preexponential factor for both components of the eclipse reaction when compared to wt virus. The activation energies for both components are the same as wt virus. These results suggest that the mechanism of the eclipse reaction can be operationally divided into two aspects, each subject to mutational alteration.

¹ Present address: Microbiology Department, University of Tennessee Medical Units, Memphis, Tenn. 38163.

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