Inactivatton of S‐rna by Ultraviolet Radiations

Various S‐RNA's from E. coli show wide differences in sensitivity to ultraviolet (u.v.) radiation as measured by their amino acid acceptor capacities. S‐RNA for valine is least sensitive and phenylalanine and lysine are most sensitive. S‐RNA's for valine, isoleucine, glycine, aspartic acid, serine, and arginine give exponential type dose‐effect curves. Those for alanine, glutamic acid, lysine, and phenylalanine are rapidly inactivated at low doses and only at higher doses give exponential type inactivation curves having lower slopes than seen at low doses. Penylalanine and glutamic acid S‐RNA inactivated by 2650 or 2804 Å radiations are not reactivated by 2380 Å radiation, indicating that uracil dimers are not involved in the inactivation process. Heating of irradiated S‐RNA to bring about rehydration of uracil residues is not effective in restoring acceptor activity of phenylalanine and leucine S‐RNA. Melting curve experiments suggest that photochemical products other than uracil diniers and hydrated uracil residues are formed and result in changes in the secondary structure of S‐RNA.