Magnesium-dependent alternative foldings of active and inactive Escherichia coli tRNA(Glu) revealed by chemical probing.

A stable conformer of Escherichia coli tRNA(Glu), obtained in the absence of Mg(2+), is inactive in the aminoacylation reaction. Probing it with diethylpyrocarbonate, dimethyl sulfate and ribonuclease V1 revealed that it has a hairpin structure with two internal loops; the helical segments at both extremities have the same structure as the acceptor stem and the anticodon arm of the native conformer of tRNA(Glu)and the middle helix is formed of nucleotides from the D-loop (G15-C20:2) and parts of the T-loop and stem (G51-C56), with G19 bulging out. This model is consistent with other known properties of this inactive conformer, including its capacity to dimerize. Therefore, this tRNA requires magnesium to acquire a conformation that can be aminoacylated, as others require a post-transcriptional modification to reach this active conformation.