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tween the base composition and the melting temperature of DNA by analyzing thermal behaviors of various DNA solutions (1962), many studies have been reported on a force(s) and/or a factor(s) responsible for the stability of DNA structure and the denaturation-renaturation mechanism of the DNA molecule. Mainly used in these studies were spectroscopic techniques and scanning calorimetry. The spectroscopic technique is generally superior in sensitivity, but it is not suitable for measurements of samples in high salt concentrations and at high temperatures. In contrast, scanning calorimetry is a useful technique to study the structure, stability, and conformational changes of biological macromolecules and the interaction between biopolymers. The principal advantage of this technique is that it can display in a single run the complete thermal features of a complex system, such as ribosomal particles or intact bacterial cells, without requiring separation into each constituent fraction. We previously described thermal transition profiles of bacteriophage T4 both in permissive and in nonpermissive E. coli cells (Kawai et al., 1979). In the study, it was shown that the well-assembled phage T4 particles carried out their thermal transition at 87°C in vivo, giving rise to a single endothermic peak on their thermogram (Fig. 1c). On the contrary, constituent protein parts and nonpackaged concatenated DNA showed a broad endothermic peak at 85°C with an additional peak at 89°C (Fig. 1b). It was suggested that a difference in vivo exists between the melting modes of DNA molecules packaged into the head shell and nonpackaged DNA molecules. During successive calorimetric measurements on bacteriophage T4 infected E. coli cells, no substantial difference was found in thermograms of the second heat scan between packaged DNA and nonpackaged DNA: only a single endothermic peak at 85°C was observed (data not shown). Therefore the observed endothermic peak at 87°C obtained with assembled phage T4 particles in host bacteria may reflect, at least partially, the interaction between the packaged DNA and the protein head shell, which is irreversibly destroyed by heat. In this paper, we describe some results of calorimetric studies on bacteriophage T4 particles and its DNA melting-rewinding profiles. E. coli B40S and B were used as permissive and nonpermissive strains, respectively, for amber mutant 49 (am NG727) of phage T4. E. coli B was used also as a host strain for wild-type T4. The growth of bacteria and phage was done at 37°C with aeration. The growth medium contained 0.4% glucose, 1 mM MgSO4, M9 salts, and 1% casamino acids. Phage DNA was prepared from T4 grown in E. coli B. Harvested phages were suspended in 10 mM Tris-20 mM EDTA (pH 7.5). After 1 min incubation at 65°C in a centrifuge tube, the suspension was mixed with 1/10 volume of SDS solution (10%) and incubated at 65°C for 20 min to solubilize phage proteins. The solubilized proteins were precipitated by adding KCl solution to a final concentration of 0.5 M and cooling them for 20 min on ice; they were removed by low centrifugation. The phage DNA solution was further purified with phenol extraction. Ethanol was added to the solution to collect purified phage DNA from the boundary between the aqueous and ethanol phase. The sample specimen thus obtained had nearly no protein contamination because the observed values of OD230/OD260 and OD280/OD260 were 0.48 and 0.58, respectively. Calorimetric measurements were made by using a Thermal transition profiles of bacteriophage T4 and its DNA

Thermal transition profiles of bacteriophage T4 and its DNA.