During bacterial protein synthesis, stalled ribosomes can be rescued by tmRNA, a molecule with both tRNA and mRNA features. The tRNA region of tmRNA has sequence similarity with tRNA A l a and also has a clover-leaf structure folded similarly as in canonical tRNAs. Here we propose the L-shape of tmRNA to be stabilized by two tertiary interactions between its D- and T-loop on the basis of phylogenetic and experimental evidence. Mutational analysis clearly demonstrates a tertiary interaction between G 1 3 and U 3 4 2 . Strikingly, this in evolution conserved interaction is not primarily important for tmRNA alanylation and for binding to elongation factor Tu, but especially for a proper functioning of SmpB.