Adaptation of the <i>Romanomermis culicivorax</i> CCA-Adding Enzyme to Miniaturized Armless tRNA Substrates

• Main Authors: Oliver Hennig, Susanne Philipp, Sonja Bonin, Kévin Rollet, Tim Kolberg, Tina Jühling, Heike Betat, Claude Sauter, Mario Mörl
• Format: Article
• Language: English
• Published: MDPI AG 2020-11-01
• Series: International Journal of Molecular Sciences
• Subjects: CCA-adding enzyme; co-evolution; evolutionary plasticity; minimalized armless tRNAs; tRNA nucleotidyltransferase
• Online Access: https://www.mdpi.com/1422-0067/21/23/9047

The mitochondrial genome of the nematode <i>Romanomermis culicivorax</i> encodes for miniaturized hairpin-like tRNA molecules that lack D- as well as T-arms, strongly deviating from the consensus cloverleaf. The single tRNA nucleotidyltransferase of this organism is fully active on armless tRNAs, while the human counterpart is not able to add a complete CCA-end. Transplanting single regions of the <i>Romanomermis</i> enzyme into the human counterpart, we identified a beta-turn element of the catalytic core that—when inserted into the human enzyme—confers full CCA-adding activity on armless tRNAs. This region, originally identified to position the 3′-end of the tRNA primer in the catalytic core, dramatically increases the enzyme’s substrate affinity. While conventional tRNA substrates bind to the enzyme by interactions with the T-arm, this is not possible in the case of armless tRNAs, and the strong contribution of the beta-turn compensates for an otherwise too weak interaction required for the addition of a complete CCA-terminus. This compensation demonstrates the remarkable evolutionary plasticity of the catalytic core elements of this enzyme to adapt to unconventional tRNA substrates.