A methyl group controls conformational equilibrium in human mitochondrial tRNALys

The unmodified maturation intermediate in the biogenesis of human mitochondrial tRNA(Lys) adopts a nonfunctional conformation, resembling an extended hairpin. Single molecule fluorescence resonance energy transfer (FRET) studies reveal the additional presence of a small population in a functional cloverleaf conformation. Both conformations are in a dynamic equilibrium, interconverting on the 100 ms time scale. The post-transcriptional methylation of adenosine 9 to 1-methyladenosine is a crucial step in the biogenesis of the mature tRNA. This single methyl group, by disfavoring the nonfunctional conformation, shifts the observed equilibrium toward the functional cloverleaf and makes it available for further maturation. Here, single molecule FRET analysis makes dynamics and thermodynamics of these small RNAs conveniently accessible and elucidates the mode of action of a simple post-transcriptional modification.