Photochemistry of tobacco mosaic virus RNA. Effect of HCN.

Abstract

Abstract— In attempting to sort out possible mechanisms of photoreactivation of tobacco mosaic virus RNA (TMV‐RNA) inactivated by ultraviolet radiation (u.v.) in buffer of ionic strength 0.25, we have investigated the effect of HCN on the quantum yield for u.v. inactivation of TMV‐RNA and on the percent photoreactivation of inactivated TMV‐RNA. Some photo‐products produced by irradiation of model substances, polyuridylic acid (poly U) and polycytidylic acid (poly C), in the presence of HCN have also been studied.The ratio of the quantum yield for inactivation of TMV‐RNA in the presence of HCN to that in the absence of HCN is 1.5, under non‐photoreactivating conditions. By comparison, the ratio of the initial rates of loss of uracil residues in poly U under comparable conditions is 1.6; by contrast, the rate of loss of cytosine residues in poly C is unaffected by HCN. This similarity of ratios between poly U and TMV‐RNA suggests that two of the mechanisms of u.v. inactivation of TMV‐RNA at high ionic strength are akin to known reactions of uracil residues in poly U, i.e. hydrate and dimer formation.The photohydration reaction in poly U, as measured by the heat reversal of hydrated residues to uracil residues, is almost abolished by HCN, and the rate of dimerization, as measured by the appearance of dimer containing oligonucleotides following enzymatic hydrolysis of irradiated poly U, is reduced to half by HCN. HCN does not affect the rate of hydration of cytosine residues in poly C. Since photoreactivation of RNA inactivated in presence of HCN is only 60 per cent of that in absence of HCN it is suggested that uracil dimers are somehow involved in photoreactivation of TMV‐RNA inactivated at high ionic strength.