Cyclization of coliphage 186 DNA

Abstract

Equilibrium and kinetic studies have been made on the cyclization of coliphage 186 DNA. In 0·13 m-Na+, the melting temperature of the cohesive ends (Tm), i.e. the midpoint of the linear monomer to cyclic monomer transition, occurs at 63°C, which is 12°C above that of λb2b5c DNA in the same medium. The molecular weight of 186 DNA is less than that of λb2b5c DNA. Sedimentation and electron microscopy data give a value 19·7 × 106 daltons for the former, taking the latter as 25·8 × 106 daltons. The difference in molecular length would predict 186 DNA a value of Tm 1°C higher than that of λb2b5c. The observed increase in Tm (12°C) suggests either the cohesive ends of 186 DNA are somewhat longer (by approximately four bases per end) or richer in GC (32% higher than the GC content of λ ends), or a proper combination of the two. The rate of cyclization of 186 DNA is approximately a factor of 10 faster than the rate of cyclization of λb2b5c DNA. The difference in molecular length would only account for a factor of 1·5. The activation energy for the cyclization of 186 DNA, 24 kcal., is the same as that of λb2b5c DNA within experimental error. The large difference in rates supports the notion that the rate-controlling step of the cyclization process is a chemical reaction rather than a diffusion step.