The effect of ultraviolet irradiation on nucleic acid isolated from tobacco mosaic virus

Abstract The effects of ultraviolet irradiation, mainly of 2537-A wavelength, on RNA extracted from tobacco mosaic virus were studied. Breakage of RNA by irradiation was measured by use of the ultracentrifuge. The quantum yield for breakage of RNA in solution was calculated to be 3.5·10 −6 moles/einstein. Frozen RNA solutions, irradiated at dry-ice or liquid-nitrogen temperatures, gave a quantum yield for breakage of 8.1·10 −6 . The heating to 68–70° of RNA solutions which had been irradiated at dry-ice or liquid-nitrogen temperatures increased the apparent quantum yields for breakage. The quantum yield for material irradiated at dry-ice temperature and heated was 2.0·10 −5 , and that for material irradiated at liquid-nitrogen temperature and heated was 3.6·10 −5 . The observed variations in quantum yield can be explained in terms of a model suggested by Doty et al . They suggest that the tobacco mosaic virus RNA strand has helical regions interspersed with regions of random coiling. A. hydrogen-bonded secondary structure is believed to stabilize the helical regions. The quantum yield for biological inactivation of frozen RNA solutions irradiated at dry-ice temperature was 7·10 −4 , in reasonably good agreement with the value of 3·10 −4 reported by McLaren and Takahashi for inactivation in solution.