Amber mutants of bacteriophage <tex-math>${\rm T}_{4}{\rm D}$</tex-math> were used to study the relative radiosensitivity of the head component and of the tail component of the phage under various experimental conditions. In the presence of radical scavenging agents the protein tails were considerably less sensitive to ionizing radiation than the intact, whole phage, whereas the DNA-containing heads were slightly more sensitive than whole bacteriophage. The Do (37% survival) of anoxic tails was 900 krad and 26 krad for heads. Addition of cysteamine reduced the radiosensitivity of phage heads and whole phage but had no effect on the tails. Likewise oxygen had a protective effect on the heads and whole phage but did not alter the radiosensitivity of the tails. Irradiation had no effect on the in vitro assembly method used for measuring the heads and tails.

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