The relationship between inactivation of tobacco mosaic virus by X-rays and breakage of nucleic acid

The analytical ultracentrifuge, with u.v. absorption optics, has been used to demonstrate and measure bond breakage in the main chain of RNA strands extracted from X-irradiated TMV. When the whole virus was irradiated in solution at 0°, the rate of loss of intact RNA strands was about equal to the rate of loss of viral activity. However, the reduction in yield of extracted RNA that accompanied increasing X-ray dose, together with the probability that longer strands would be selectively lost, cast considerable doubt on the significance of this result. When TMV was irradiated while frozen at about —50° or while in centrigugal pellets with little water present, the correlation between reduction in yield and X-ray dose disappeared. Those data indicate a D 0 for strand breakages of 4.0·10 5 R as compared to an average measured D 0 of 1.8·10 5 R for virus inactivation. Thus, breakage proceeds 40% as rapidly as does inactivation. Also, 40% of all the ionizations in TMV RNA induced by radiation may be expected to occur among backbone chain electrons. If it is assumed 1 that any ionization within the nucleic acid complement of a virus inactivates the virus, then the present data indicate that ionizations within the backbone chain of TMV RNA break main chain bonds and cleave the strand with an efficiency of close to 100%. By analogy, then, it is suggested that ionizations elsewhere in the nucleic acid also are effective in breaking bonds.