Gene function and strand breaks in DNA of gamma-irradiated T7 bacteriophages

Induction of mutations in bacteriophage T7 by gamma-rays: independence of host-repair mechanisms

Amber mutants of bacteriophage T7 are reverted by gamma-rays to pseudo wild-type particles, i.e. particles able to propagate in a suppressorless host. The yield of revertants is much higher when the phage is irradiated in the presence of oxygen than when irradiated anoxically. Under particular gas conditions the efficiency of mutation induction differs by less than a factor of ten among six different amber codons in cistrons 1, 5, 6, 12, 17 and 19. The induction of mutations is not dependent on error-prone repair involving the recA or lexA genes of the host cell. It is estimated that of the damages that may be inflicted by gamma-rays upon an amber codon, fewer than 1 out of 85 results in reversion of the codon to pseudo wild-type.

The effect of a mammalian repair endonuclease on x-irradiated DNA

DNA was extracted from rat liver of non-irradiated animals, and was irradiated in vitro, and from animals which received whole body doses of X-radiation. Sedimentation on neutral and alkaline sucrose gradients as well as measurements of 32P release after sequential treatment with endonuclease and alkaline phosphatase and determination of triphosphate incorporation after the sequential treatment with endonuclease, alkaline phosphatase and DNA polymerase indicated that DNA irradiated in vivo and in vitro were effective substrates for the mammalian repair endonuclease. The experiments suggest that in addition to strand breaks, X-radiation causes base damage and they have provided a plausible explanation for the formation of double strand breaks in DNA irradiated in vivo.