Effects of Ultraviolet Radiation on RNA C-C Glycosyls and Other Derivatives of Uracil

Abstract

After ultraviolet irradiation many pyrimidine derivatives do not initially undergo ring rupture but take up a molecule of water at the 5,6 double bond (hydration). Subsequently, under the influence of acid, alkali, or heating at neutral pH, the photoproduct may eliminate the molecule of water and revert to its initial unsaturation (1). Studies of the rates of hydration of pyrimidines (substituted at positions 1 and 1,5) show that both the nature of the groups and the positions of substitution by the groups influence the rates of hydration (1, 2). Although the function of 1,5-diribosyluracil (3) has not yet been elucidated, the fact that it is found in trace amounts in human urine (4) indicates that it is a common metabolite of mammalian tissue. 5-Ribosyluracil (5) is definitely one of the components that characterize the unpaired portions of sRNA (6), and it is a component of the pentonucleotide (-GpTp5URpCpGp-), common to all yeast, a few bacteria, and some liver transfer ribonucleic acids (7). Fawaz-Estrup and Setlow, in their studies on inactivation of valine-s-RNA by ultraviolet radiation, concluded that only the absorption of photons in the uridine and 5-ribosyluracil residues is responsible for the loss of the biological specificity of s-RNA (8). According to available evidence (6), those areas are unpaired and are referred to as single-stranded loops. The results of a present investigation indicate that the C-C glycosyls, such as 5-ribosyluracil and 1,5-diribosyluracil, are not subject to complete reactivation. Photochemical irreversibility of C-C glycosyls found in RNA may be of major importance, since this fact may be responsible for nongenetic errors in protein biosynthesis (2, 9).