Thermal inactivation of bacteriophage ∅X174 and two of its mutants

Abstract

The thermal inactivation of the two forms (∅∗ and ∅) of bacteriophage ∅X174 with different adsorption properties was investigated under various conditions of pH, ionic strength, and temperature. A reaction mechanism is proposed that is similar for both forms. At high temperatures inactivation occurs mainly as a result of protein destruction and is dependent on the structure of the adsorption site(s). At lower temperatures the contribution of DNA destruction is higher. In addition to a difference in heat sensitivity, the two forms show a different dependence of their inactivation rate on pH and ionic strength. Application of the theory of absolute reaction rates, using the estimates of Stearn concerning enthalpy and entropy of activation for bonds broken during denaturation of proteins, suggests that the structural difference between ∅∗ and ∅ may be determined by only one weak noncovalent bond. The Arrhenius curves of the host-range mutant ∅X174 h1 h2 and of the thermoresistant mutant ∅X174 st are similar to that of the wild type. Comparison of the Arrhenius curves of these three phages and of their rate of transition from the ∅∗ into the ∅ form suggests that the bonds primarily involved in this transition are not located in the immediate vicinity of the adsorption site(s).