A bacteriophage (TØ3) which infects the thermophilic bacteriumBacillus stearothermophilus ATCC 8005 was isolated and characterized.Infection of the bacterium by the bacteriophage was carried out at 60°C,the optimum growth temperature of the host. At 60°C the phage has alatent period of 18 minutes and a burst size of about 200. The phageis comparatively thermostable in broth. The half life of the phage is400 minutes at 60°C, 120 minutes at 65°C, 40 minutes at 70°C and 12minutes at 75°C. The activation energy for the heat inactivation ofTØ3 is 56,000 cal. The buoyant density of TØ3 in a cesium chloridedensity gradient is 1.526.
Electron micrographs of TØ3 indicate that the phage has a regularhexagonal shaped head 57 mμ long. The morphology of the head iscompatible with icosahedral symmetry. Each edge of the head is 29 mμlong, and there are 6 or 7 subunits along each edge. The tail of TØ3is 125 mμ long and 10 mμ wide. There are about 30 cross striations thatare spaced at 3.9 mμ intervals along the tail.
The DNA of phage TØ3 has a melting temperature of 88.5°C. Heatdenatured TØ3 DNA can be extensively annealed in a high ionic strengthenvironment. The buoyant density of TØ3 DNA in a cesium chloridedensity gradient is 1.695. TØ3 DNA contains: 42.7% guanine plus cytosine,as determined from the melting temperature; 43% guanine pluscytosine, as determined from the buoyant density; and 40.2% guanineplus cytosine, as determined by chromatographic separation and spectrophotometricestimation of the bases. The molecular weight of TØ3DNA is 16.7 X 106 as determined from the band width of the TØ3 DNAconcentration distribution in a cesium chloride density gradient. Electronmicroscopy of TØ3 DNA revealed a single linear molecule that is11.7 μ long. This corresponds to a molecular weight of 22.5 X 106.
Heat denatured TØ3 DNA forms two bands in a cesium chloride densitygradient, one at a density of 1.707 and the other at a density of 1.715.After the separated bands are mixed and annealed in the centrifuge cell,the renatured TØ3 DNA forms a single band at a density of 1.699. Theseresults indicate that the two complementary strands of TØ3 DNA havedifferent buoyant densities in cesium chloride, presumably because theyhave different base compositions.
The characteristics of TØ3 are compared with those of other phages.A hypothesis is presented for a relationship between the base compositionof one strand of TØ3 DNA and the amino acid composition of the proteinsof TØ3.
