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Transfer RNA travels from the cytoplasm to organelles

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Abstract Transfer RNAs (tRNAs) encoded by the nuclear genome are surprisingly dynamic. Although tRNAs function in protein synthesis occurring on cytoplasmic ribosomes, tRNAs can transit from the cytoplasm to the nucleus and then again return to the cytoplasm by a process known as the tRNA retrograde process. Subsets of the cytoplasmic tRNAs are also imported into mitochondria and function in mitochondrial protein synthesis. The numbers of tRNA species that are imported into mitochondria differ among organisms, ranging from just a few to the entire set needed to decode mitochondrially encoded mRNAs. For some tRNAs, import is dependent on the mitochondrial protein import machinery, whereas the majority of tRNA mitochondrial import is independent of this machinery. Although cytoplasmic proteins and proteins located on the mitochondrial surface participating in the tRNA import process have been described for several organisms, the identity of these proteins differ among organisms. Likewise, the tRNA determinants required for mitochondrial import differ among tRNA species and organisms. Here, we present an overview and discuss the current state of knowledge regarding the mechanisms involved in the tRNA retrograde process and continue with an overview of tRNA import into mitochondria. Finally, we highlight areas of future research to understand the function and regulation of movement of tRNAs between the cytoplasm and organelles. WIREs RNA 2011 2 802–817 DOI: 10.1002/wrna.93 This article is categorized under: RNA Processing > tRNA Processing RNA Export and Localization > Nuclear Export/Import RNA Export and Localization > RNA Localization

Movement of transfer RNAs (tRNAs) in eukaryotic cells. Nuclear‐encoded tRNAs are transcribed and largely processed in the nucleus. In yeast, but not in vertebrate cells, tRNAs encoded by intron‐containing genes, are exported (initial tRNA export; teal arrow) to the cytoplasm prior to being spliced. Splicing occurs on the outer surface of mitochondria. tRNAs in the cytoplasm are able to return to the nucleus via retrograde import (orange arrow) and, under nutrient replete conditions, the tRNAs can be re‐exported to the cytoplasm (blue arrow). A subset of tRNAs can be imported into the mitochondrial matrix, past the outer and inner mitochondrial membranes (purple arrows). It is unknown whether cytoplasmic tRNAs are imported into other organelles such as chloroplasts (gray arrow).

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Sequence determinants for transfer RNA (tRNA) import into mitochondria: (a) anticodon nucleotide of T. pyriformis tRNAGln; (b) anticodon and D arm of N. tabaccum tRNAGly; (c) anticodon, D arm and TΨC arm of N. tabaccum tRNAVal; (d) intergenic sequence between the precursor form of the dicistronic tRNASer and tRNALeu of T. brucei; (e) T‐stem base pair 51:63 of T. brucei tRNA; (f) D arm of L. tarentolae tRNAIle and tRNAGln; (g) D arm of L. tropica tRNATyr; (h) anticodon, acceptor stem base pair 1:72, acceptor nucleotide, 73 of S. cerevisiae tRNALys(CUU); and (i) proposed conformational rearrangement of S. cerevisiae tRNALys(CUU) into the F‐structure. Dark lines highlight the sequence determinant(s).

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Two types of transfer RNA (tRNA) import into mitochondria. Type A: import is strictly dependent on the protein import pathway (a) S. cerevisiae tRNALys(CUU) (tRK1) is aminoacylated then recognized by enolase the precursor form of the mitochondrial lysyl‐tRNA synthetase followed by delivery to the mitochondrial surface type B: tRNA import occurs independently from the protein import pathway. (b) S. cerevisiae tRNAGln, human tRNAGln, and L. tarentolae tRNAs, (c) L. tropica tRNATyr, (d) T. brucei tRNA, (e) plant tRNAGln.

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Browse by Topic

RNA Export and Localization > Nuclear Export/Import
RNA Export and Localization > RNA Localization
RNA Processing > tRNA Processing

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