The amount of uracil dimer formed in yeast transfer RNA by ultraviolet (253.7 mμ) irradiation was determined as a function of dose. It showed a transient increase followed by a subsequent gradual decrease, as expected. The cross section of dimer formation was estimated to be$17\times 10^{-8}\ {\rm cm}^{2}/{\rm erg}$. The kinetics of the uracil photoreaction other than dimer were determined as the total change in uracil minus dimer and were found to be first order. The cross section of this reaction, most likely hydration, was evaluated to be$4.0\times 10^{-8}\ {\rm cm}^{2}/{\rm erg}$.

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