Although both tRNALya and tRNAGlu of tRNALya possess similar anticodon loop sequences, with the same hypermodifled nucleoside 5-methylaminomethyl-2-thiouridine (mnm582U) at the first position of their anticodons, the anticodon loop structures of these two tRNAs containing the modified nucleoside appear to be quite different as judged from the following observations. (1) The CD band derived from the mnm5s2U residue is negative for tRNAGlu, but positive for tRNALya. (2) The mnm5s2U monomer itself and the mnm5s2U-containing anticodon loop fragment of tRNALya show the same negative CD bands as that of tRNAGlu. (3) The positive CD band of tRNALya changes to negative when the temperature is raised. (4) The reactivity of the mnm5s2U residue toward H2O2 is much lower for tRNALya than for tRNAGlu. These features suggest that tRNALya has an unusual anticodon loop structure, in which the mnm5s2U residue takes a different conformation from that of tRNAGlu; whereas the mnm5s2U base of tRNAGlu has no direct bonding with other bases and is accessible to a solvent, that of tRNALya exists as if in some way buried in its anticodon loop. The limited hydrolysis of both tRNAs by various RNases suggests that some differences exist in the higher order structures of tRNALya and tRNAGlu. The Influence of the unusual anticodon loop structure observed for tRNALya on its function in the translational process is also discussed.
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