Abstract

The mitochondrial genome of the nematode Romanomermis culicivorax 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 Romanomermis 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.

Highlights

  • TRNAs are the essential adaptor molecules which enable the decoding of the nucleic acid code into the amino acid sequence during the translational process [1]

  • As it was reported that the CCA-adding enzyme of Caenorhabditis elegans is adapted to the bizarre mitochondrial tRNAs of this organism [52], the sequence of this enzyme as well as that of Ascaris suum were included (Figure 1)

  • A second function of CCA-adding enzyme is to monitor the intactness of its tRNA substrate, so that only undamaged molecules are accepted for CCA-incorporation [72,73,74,75,76]

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Summary

Introduction

TRNAs are the essential adaptor molecules which enable the decoding of the nucleic acid code into the amino acid sequence during the translational process [1]. To fulfill this function, they need to undergo several maturation steps and interact with the translational machinery [2,3,4,5]. The 3 -terminal CCA-triplet of the acceptor stem is a prerequisite for aminoacylation and the correct positioning of the charged tRNA in the ribosome [11,12]. This triplet is not genomically encoded but is added post-transcriptionally by tRNA nucleotidyltransferase (CCA-adding enzyme) [13,14,15]

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