Importance of conserved residues for the conformation of the T-loop in tRNAs.

The conformation of the T-loop of yeast tRNA(Asp) was studied by structural mapping techniques using chemical and enzymatic probes and by three-dimensional graphics modeling with the known crystallographic structures of tRNAs as references. The structural importance of C61 (conserved in the T-stem of all tRNAs) for the loop conformation was directly checked by ethylnitrosourea phosphate alkylation, either on the 3'-half tRNAAsp molecule or on a variant in which C61 was replaced by U61. The reactivity of P60 against ethylnitrosourea alkylation in the variant emphasizes the role of the hydrogen bond between this phosphate and position N4 of C61 for stabilizing the conformation of the T-loop. Experiments on several tRNA variants, containing C61 but altered in the sequence or in the length of the T-loop, indicate that other structural features help to stabilize the hydrogen bond network around P60. Evidence is presented that the reverse Hoogsteen base pair T54-A58 contributes to this stabilization by maintaining the hydrogen bonding between the 2'OH of ribose 58 and P60. Using graphics modeling and based on the chemical data. T-loops of several variants were constructed. It appears that both the constant length of the T-loop and the presence of psi 55 are crucial for the correct interaction between the T- and D-loops. The conclusion of this study is that the T-loop in tRNA possesses an intrinsic conformation (mainly governed by the constant residues) existing primarily without the structural context of the entire tRNA molecule.