Please do not recycle! Translation reinitiation in microbes and higher eukaryotes

Abstract

Protein production must be strictly controlled at its beginning and end to synthesize a polypeptide that faithfully copies genetic information carried in the encoding mRNA. In contrast to viruses and prokaryotes, the majority of mRNAs in eukaryotes contain only one coding sequence, resulting in production of a single protein. There are, however, many exceptional mRNAs that either carry short open reading frames upstream of the main coding sequence (uORFs) or even contain multiple long ORFs. A wide variety of mechanisms have evolved in microbes and higher eukaryotes to prevent recycling of some or all translational components upon termination of the first translated ORF in such mRNAs and thereby enable subsequent translation of the next uORF or downstream coding sequence. These specialized reinitiation mechanisms are often regulated to couple translation of the downstream ORF to various stimuli. Here we review all known instances of both short uORF-mediated and long ORF-mediated reinitiation and present our current understanding of the underlying molecular mechanisms of these intriguing modes of translational control.

Figure 1.

Model of the entire translational cycle with two basic ways of translation reinitiation: (i) the 40S-mediated REI after short versus long uORFs and (ii) the 80S-mediated REI after long ORFs. For details, see the main text (adapted from Valasek et al. 2017).

Figure 2.

(A) Model of the 5^΄ leader of GCN4 mRNA with its four short uORFs summarizing all REI-promoting and inhibiting RNA and protein features (adapted from Gunisova et al.2016). For details, see the main text. (B) Graphical illustration of the proposed arrangement of the post-termination complex on uORF1 with its RPEs interacting with Box 6 and Box 17 segments of the N-terminal domain of a/Tif32 to promote resumption of scanning for REI on GCN4 (adopted from Mohammad et al.2017). The exit channel view of the 48S closed complex shows only two incomplete eIF3 subunits for simplicity: eIF3c/Nip1 in light gold and eIF3a/Tif32 in purple, with its CTD represented by a dotted line (the structure of this domain is unknown and thus its placement in the 48S complex was only predicted). The location of Boxes 6 + 17 is indicated in green. The 5^΄ leader of uORF1 is shown in orange with its RPEs depicted in yellow. The predicted position of eIF3g/Tif35 is indicated by the blue circle. (C) Model of the ATF4 mRNA; RPEs surrounding uORF1 are depicted in green and the prospective interaction between eIF3 and the 5´ RPE is indicated. For details, see the main text.

Figure 3.

Model for GCN4 translational control under non-starvation versus starvation conditions mediated via reinitiation in response to changing levels of the eIF2-TC. For details, see the main text (adapted from Gunisova and Valasek 2014).

Figure 4.

Model for translational control of short and long uORFs-containing mRNAs mediated via reinitiation promoted by eIF3h phosphorylation in A. thaliana. For details, see the main text.

Figure 5.

Model for translational control of short uORFs or start-stop uORFs-containing mRNAs mediated via reinitiation promoted by DENR-MCT-1 as the REI-specific factors. For details, see the main text.

Figure 6.

(A) Model for the termination-reinitiation mechanism in caliciviruses with overlapping ORFs mediated via TURBS base pairing with 18S rRNA, which can be perhaps further potentiated by eIF3 binding to both TURBS and the 40S. For details, see the main text. (B) Predicted secondary structure of the FCV TURBS illustrating base pairing between motif 1 and helix 26 of 18S rRNA, motifs 2 and 2^*, as well as the schematic interaction of TURBS with eIF3 (based on Jackson, Hellen and Pestova 2012).

Figure 7.

Model for translational control of the CaMV mRNA mediated via reinitiation promoted by the virus-encoded transactivator protein called TAV and the plant-specific REI supporting protein (RISP). For details, see the main text.

Figure 8.

Model for translation reinitiation in the 3´ UTR. For details, see the main text.

Figure 9.

Model for translation reinitiation in prokaryotes; SD—Shine-Dalgarno sequence; RRF—ribosome recycling factor. For details, see the main text.

Figure 10.

Model for retroreinitiation within the coding region; PTC—premature termination codon. For details, see the main text.

Figure 11.

Model for the StopGo translation reinitiation. For details, see the main text.