Effect of X-rays on nucleic acid isolated from tobacco mosaic virus

Breakage of RNA, X-irradiated after phenol extraction from tobacco mosaic virus, was measured using the ultracentrifuge equipped with u.v. absorption optics. The sedimentation patterns of control solutions of RNA show a homogeneous component corresponding in size to the fraction of monomer in the parent virus. By following the decrease of this RNA component with increasing X-ray dose, reproducible values for the 37 % survival dose, D 0, for breakage of the main chain can be obtained. The indirect action of radiation was minimized by irradiating RNA in concentrated, oxygen-free, frozen solution. Under these conditions the D 0 for breakage was 21·10 5 R, while D 0 for dried RNA was 4.2·10 5 R, and for RNA in the virus was 4.0·10 5 R (see ref. 1). The anomaly could be due to intramolecular hydrogen bonding 2, since heating the irradiated RNA solutions to 65 or 70° drastically increased breakage leading to an effective D 0 of 4.4·10 5 R.