Transfer RNA, II. A structural basis for alanine acceptor activity.

In an effort to elucidate the chemical basis for recognition of a transfer RNA (tRNA) by its aminoacyl synthetase, we have been studying the inactivation of purified tRNAala Iab (ref. 1) by ultraviolet light. There are four known photoreactions that might cause inactivation of tRNA:3 (1) photohydration of C and U residues; (2) dimerization of adjacent C and U residues; (3) chain cleavage at VI residues;4 (4) modification of i+ residues without chain cleavage.5 We investigated the pseudouridine cleavage reaction in detail and found that inactivation of alanine acceptor activity can occur without chain cleavage.4' 5 In this paper, we shall confine ourselves to inactivation of intact molecules. The potential targets for these photoreactions are shown in Figure 1. These targets are not equally sensitive, however. The reaction cross sections for the four above reactions are different at the nucleotide level, and ordered structure alters the reaction cross sections still further in nucleic acids.6-8 It is not surprising, then, that the irradiation conditions play an important role on the photochemistry of tRNAala Iab In dilute salt, inactivation of alanine acceptor activity is rapid and complex.9 The inactivation shows a D20 isotope effect of 1.8, an indication that photohydration is an inactivating event.9 Although the photoproducts formed under these conditions have not been examined further, it is probable that most of the targets shown in Figure 1 react.