Unraveling the landscapes and regulation of scanning, leaky scanning, and 48S initiation complex conformations

Scanning and initiation are critical steps in translation. Here, we utilized translation complex profiling (TCP-seq) to investigate 48S organization and eIF4G1-eIF1 inhibition impact. We provide global views of scanning and leaky scanning, uncovering a central role of eIF4G1-eIF1 in their regulation. We confirm AUG context importance, with non-leaky genes featuring a Kozak context and cytosine at positions -1 and +5. Capturing 48S complexes associated with eIF1, eIF4G1, eIF3, and eIF2 through selective TCP-seq revealed that the eIF3-scanning ribosome is highly vulnerable to eIF4G1-eIF1 inhibition, and eIF1 tends to dissociate upon AUG recognition. Initiation-site footprint analysis revealed a class spanning -12 to +18/19 from the AUG, representing the entire 48S and enriched with eIF2, eIF1, and eIF4G1, indicative of early initiation. Another eIF3-dependent class extends up to +26 and exhibits reduced eIF2 and eIF4G1 association, suggesting a late/alternative initiation complex. Our analysis provides an overview of scanning, initiation, and evidence for conformational rearrangements in vivo.

Short 5' Untranslated Region Enables Optimal Translation of Plant Virus Tricistronic RNA via Leaky Scanning

Regardless of the general model of translation in eukaryotic cells, a number of studies suggested that many mRNAs encode multiple proteins. Leaky scanning, which supplies ribosomes to downstream open reading frames (ORFs) by readthrough of upstream ORFs, has great potential to translate polycistronic mRNAs. However, the mRNA elements controlling leaky scanning and their biological relevance have rarely been elucidated, with exceptions such as the Kozak sequence. Here, we have analyzed the strategy of a plant RNA virus to translate three movement proteins from a single RNA molecule through leaky scanning. The in planta and in vitro results indicate thatthe significantly shorter 5' untranslated region (UTR) of the most upstream ORF promotes leaky scanning, potentially fine-tuning the translation efficiency of the three proteins in a single RNA molecule to optimize viral propagation. Our results suggest that the remarkably short length of the leader sequence, like the Kozak sequence, is a translational regulatory element with a biologically important role, as previous studies have shown biochemically. IMPORTANCE Potexvirus, a group of plant viruses, infect a variety of crops, including cultivated crops. It has been thought that the three transition proteins that are essential for the cell-to-cell transfer of potexviruses are translated from two subgenomic RNAs, sgRNA1 and sgRNA2. However, sgRNA2 has not been clearly detected. In this study, we have shown that sgRNA1, but not sgRNA2, is the major translation template for the three movement proteins. In addition, we determined the transcription start site of sgRNA1 in flexiviruses and found that the efficiency of leaky scanning caused by the short 5' UTR of sgRNA1, a widely conserved feature, regulates the translation of the three movement proteins. When we tested the infection of viruses with mutations introduced into the length of the 5' UTR, we found that the movement efficiency of the virus was affected. Our results provide important additional information on the protein translation strategy of flexiviruses, including Potexvirus, and provide a basis for research on their control as well as the need to reevaluate the short 5' UTR as a translational regulatory element with an important role in vivo.

The rax homeobox gene is mutated in the eyeless axolotl, Ambystoma mexicanum

BACKGROUND

Vertebrate eye formation requires coordinated inductive interactions between different embryonic tissue layers, first described in amphibians. A network of transcription factors and signaling molecules controls these steps, with mutations causing severe ocular, neuronal, and craniofacial defects. In eyeless mutant axolotls, eye morphogenesis arrests at the optic vesicle stage, before lens induction, and development of ventral forebrain structures is disrupted.

RESULTS

We identified a 5-bp deletion in the rax (retina and anterior neural fold homeobox) gene, which was tightly linked to the recessive eyeless (e) axolotl locus in an F2 cross. This frameshift mutation, in exon 2, truncates RAX protein within the homeodomain (P154fs35X). Quantitative RNA analysis shows that mutant and wild-type rax transcripts are equally abundant in E/e embryos. Translation appears to initiate from dual start codons, via leaky ribosome scanning, a conserved feature among gnathostome RAX proteins. Previous data show rax is expressed in the optic vesicle and diencephalon, deeply conserved among metazoans, and required for eye formation in other species.

CONCLUSION

The eyeless axolotl mutation is a null allele in the rax homeobox gene, with primary defects in neural ectoderm, including the retinal and hypothalamic primordia.

Leaky scanning translation generates a second A49 protein that contributes to vaccinia virus virulence

Vaccinia virus (VACV) strain Western Reserve gene A49L encodes a small intracellular protein with a Bcl-2 fold that is expressed early during infection and has multiple functions. A49 co-precipitates with the E3 ubiquitin ligase β-TrCP and thereby prevents ubiquitylation and proteasomal degradation of IκBα, and consequently blocks activation of NF-κB. In a similar way, A49 stabilizes β-catenin, leading to activation of the wnt signalling pathway. However, a VACV strain expressing a mutant A49 that neither co-precipitates with β-TrCP nor inhibits NF-κB activation, is more virulent than a virus lacking A49, indicating that A49 has another function that also contributes to virulence. Here we demonstrate that gene A49L encodes a second, smaller polypeptide that is expressed via leaky scanning translation from methionine 20 and is unable to block NF-κB activation. Viruses engineered to express either only the large protein or only the small A49 protein both have lower virulence than wild-type virus and greater virulence than an A49L deletion mutant. This demonstrates that the small protein contributes to virulence by an unknown mechanism that is independent of NF-κB inhibition. Despite having a large genome with about 200 genes, this study illustrates how VACV makes efficient use of its coding potential and from gene A49L expresses a protein with multiple functions and multiple proteins with different functions.

Dynamic Interaction of Eukaryotic Initiation Factor 4G1 (eIF4G1) with eIF4E and eIF1 Underlies Scanning-Dependent and -Independent Translation

Translation initiation of most mRNAs involves m7G-cap binding, ribosomal scanning, and AUG selection. Initiation from an m7G-cap-proximal AUG can be bypassed resulting in leaky scanning, except for mRNAs bearing the translation initiator of short 5' untranslated region (TISU) element. m7G-cap binding is mediated by the eukaryotic initiation factor 4E (eIF4E)-eIF4G1 complex. eIF4G1 also associates with eIF1, and both promote scanning and AUG selection. Understanding of the dynamics and significance of these interactions is lacking. We report that eIF4G1 exists in two complexes, either with eIF4E or with eIF1. Using an eIF1 mutant impaired in eIF4G1 binding, we demonstrate that eIF1-eIF4G1 interaction is important for leaky scanning and for avoiding m7G-cap-proximal initiation. Intriguingly, eIF4E-eIF4G1 antagonizes the scanning promoted by eIF1-eIF4G1 and is required for TISU. In mapping the eIF1-binding site on eIF4G1, we unexpectedly found that eIF4E also binds it indirectly. These findings uncover the RNA features underlying regulation by eIF4E-eIF4G1 and eIF1-eIF4G1 and suggest that 43S ribosome transition from the m7G-cap to scanning involves relocation of eIF4G1 from eIF4E to eIF1.

Ribosome Shunting, Polycistronic Translation, and Evasion of Antiviral Defenses in Plant Pararetroviruses and Beyond

Viruses have compact genomes and usually translate more than one protein from polycistronic RNAs using leaky scanning, frameshifting, stop codon suppression or reinitiation mechanisms. Viral (pre-)genomic RNAs often contain long 5′-leader sequences with short upstream open reading frames (uORFs) and secondary structure elements, which control both translation initiation and replication. In plants, viral RNA and DNA are targeted by RNA interference (RNAi) generating small RNAs that silence viral gene expression, while viral proteins are recognized by innate immunity and autophagy that restrict viral infection. In this review we focus on plant pararetroviruses of the family Caulimoviridae and describe the mechanisms of uORF- and secondary structure-driven ribosome shunting, leaky scanning and reinitiation after translation of short and long uORFs. We discuss conservation of these mechanisms in different genera of Caulimoviridae, including host genome-integrated endogenous viral elements, as well as in other viral families, and highlight a multipurpose use of the highly-structured leader sequence of plant pararetroviruses in regulation of translation, splicing, packaging, and reverse transcription of pregenomic RNA (pgRNA), and in evasion of RNAi. Furthermore, we illustrate how targeting of several host factors by a pararetroviral effector protein can lead to transactivation of viral polycistronic translation and concomitant suppression of antiviral defenses. Thus, activation of the plant protein kinase target of rapamycin (TOR) by the Cauliflower mosaic virus transactivator/viroplasmin (TAV) promotes reinitiation of translation after long ORFs on viral pgRNA and blocks antiviral autophagy and innate immunity responses, while interaction of TAV with the plant RNAi machinery interferes with antiviral silencing.

Close spacing of AUG initiation codons confers dicistronic character on a eukaryotic mRNA

TYMV RNA supports the translation of two proteins, p69 and p206, from AUG initiation codons 7 nucleotides apart. We have studied the translation of this overlapping dicistronic mRNA with luciferase reporter RNAs electroporated into cowpea protoplasts and in toe-printing studies that map ribosomes stalled during initiation in wheat germ extracts. Agreement between these two assays indicates that the observed effects reflect ribosome initiation events. The robust expression from the downstream AUG206 codon was dependent on its closeness to the upstream AUG69 codon. Stepwise separation of these codons resulted in a gradual increase in upstream initiation and decrease in downstream initiation, and expression was converted from dicistronic to monocistronic. Selection by ribosomes for initiation between the nearby AUG codons was responsive to the sequence contexts that govern leaky scanning, but the normally strong position effect favoring upstream initiation was greatly diminished. Similar dicistronic expression was supported for RNAs with altered initiation sequences and for RNAs devoid of flanking viral sequences. Closely spaced AUG codons may thus represent an under-recognized strategy for bicistronic expression from eukaryotic mRNAs. The initiation behavior observed in these studies suggests that 5'-3' ribosome scanning involves backward excursions averaging about 15 nucleotides.

Pushing the limits of the scanning mechanism for initiation of translation

Selection of the translational initiation site in most eukaryotic mRNAs appears to occur via a scanning mechanism which predicts that proximity to the 5' end plays a dominant role in identifying the start codon. This "position effect" is seen in cases where a mutation creates an AUG codon upstream from the normal start site and translation shifts to the upstream site. The position effect is evident also in cases where a silent internal AUG codon is activated upon being relocated closer to the 5' end. Two mechanisms for escaping the first-AUG rule--reinitiation and context-dependent leaky scanning--enable downstream AUG codons to be accessed in some mRNAs. Although these mechanisms are not new, many new examples of their use have emerged. Via these escape pathways, the scanning mechanism operates even in extreme cases, such as a plant virus mRNA in which translation initiates from three start sites over a distance of 900 nt. This depends on careful structural arrangements, however, which are rarely present in cellular mRNAs. Understanding the rules for initiation of translation enables understanding of human diseases in which the expression of a critical gene is reduced by mutations that add upstream AUG codons or change the context around the AUG(START) codon. The opposite problem occurs in the case of hereditary thrombocythemia: translational efficiency is increased by mutations that remove or restructure a small upstream open reading frame in thrombopoietin mRNA, and the resulting overproduction of the cytokine causes the disease. This and other examples support the idea that 5' leader sequences are sometimes structured deliberately in a way that constrains scanning in order to prevent harmful overproduction of potent regulatory proteins. The accumulated evidence reveals how the scanning mechanism dictates the pattern of transcription--forcing production of monocistronic mRNAs--and the pattern of translation of eukaryotic cellular and viral genes.

Transactivation-defective c-MycS retains the ability to regulate proliferation and apoptosis

Transcriptional activation by c-Myc through specific E box elements is thought to be essential for its biological role. However, c-MycS is unable to activate transcription through these elements and yet retains the ability to stimulate proliferation, induce anchorage-independent growth, and induce apoptosis. In addition, c-MycS retains the ability to repress transcription of several specific promoters. Furthermore, c-MycS can rescue the c-myc null phenotype in fibroblasts with homozygous deletion of c-myc. Taken together, our data argue against the paradigm that all of the biological functions of c-Myc are mediated by transcriptional activation of specific target genes through E box elements.

Identification and analysis of frameshift sites

There are several ways that genes may encode alternative products. The most widely recognized mechanism is alternative splicing. However, genes may also employ noncanonical translational events to produce such products. Some of these mechanisms operate at the level of translational initiation. In prokaryotes, genes may include alternative ribosome-binding sites directing the synthesis of products that differ at the N terminus. In eukaryotes, in which ribosome-binding sites do not exist, leaky scanning allows the same kind of variation. Noncanonical elongation events can also generate products that differ at their C terminus (1–3). Such events include programmed readthrough of translational termination codons (4,5) translational frameshifts (6–9), and translational hops (10,11). In each case, the ribosome fails to follow normal rules of decoding, leading to the synthesis of a protein that is not encoded, in the normal sense, in the DNA. In this chapter, we will describe the methods employed in the identification and analysis of programmed translational frameshift sites, including their discovery, measurement of the efficiency of the events, and determination of the mechanism of the frameshift.