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 o f Mg2+, is inactive in the aminoacylation reaction. Probing it with diethyl pyrocarbonate, dimethyl sulfate and ribonuclease V1 revealed that it has a hairpin structure with two internal loops; the helical segments at both ext remities 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 nuc leotides 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 prope rties of this inactive conformer, including its capacity to dimerize, There fore, this tRNA requires magnesium to acquire a conformation that can be am inoacylated, as others require a posttranscriptional modification to reach this active conformation.