Abstract
During maturation, tRNA molecules undergo a series of individual processing steps, ranging from exo- and endonucleolytic trimming reactions at their 5'- and 3'-ends, specific base modifications and intron removal to the addition of the conserved 3'-terminal CCA sequence. Especially in mitochondria, this plethora of processing steps is completed by various editing events, where base identities at internal positions are changed and/or nucleotides at 5'- and 3'-ends are replaced or incorporated. In this review, we will focus predominantly on the latter reactions, where a growing number of cases indicate that these editing events represent a rather frequent and widespread phenomenon. While the mechanistic basis for 5'- and 3'-end editing differs dramatically, both reactions represent an absolute requirement for generating a functional tRNA. Current in vivo and in vitro model systems support a scenario in which these highly specific maturation reactions might have evolved out of ancient promiscuous RNA polymerization or quality control systems.
Highlights
TRNAs play a central role as adapter molecules, linking the genetic information encoded in mRNA to the amino acid sequence of the encoded protein
RNAs (e.g., A to I or C to U editing) [3]. These results indicated that this tRNA editing event must utilize a distinct mechanism from the deamination type of editing that was commonly found in other lower eukaryotes
The base portion of the nucleotide is modified without breaking any phosphodiester bonds, and the requirement for C to U editing as a prerequisite for precursor processing in plant mitochondria suggests a similar deaminase-catalyzed mechanism occurring on the unprocessed precursor transcript
Summary
TRNAs play a central role as adapter molecules, linking the genetic information encoded in mRNA to the amino acid sequence of the encoded protein. The editing process in these organisms appeared to be similar to what was observed in A. castellanii, and mismatches were restricted to the first three base pairs on the acceptor stem In all of these cases, 5'-editing appeared to be coupled with other 5'-end processing events, implied by the fact that relatively few partially edited intermediates have been obtained from the sequencing experiments of these mitochondrial encoded tRNAs [13,30,33]. The identification of the first enzymes likely to participate in 5'-editing of tRNA resulted from characterization of Thg orthologs from Bacteria and Archaea, which had been named Thg1-like proteins (TLPs) [39] These TLPs exhibited key biochemical differences from the prototypical eukaryotic Thg, in that they prefer to catalyze template-dependent 3'–5'-nucleotide addition, and importantly, have the ability to repair 5'-truncated tRNAs in a templated manner [40,41,42]. The precise roles of each of these enzymes in the tRNA 5'-editing reaction awaits future in vivo characterization
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