Pausing on Polyribosomes: Make Way for Elongation in Translational Control

Among the three phases of mRNA translation—initiation, elongation, and termination—initiation has traditionally been considered to be rate limiting and thus the focus of regulation. Emerging evidence, however, demonstrates that control of ribosome translocation (polypeptide elongation) can also be regulatory and indeed exerts a profound influence on development, neurologic disease, and cell stress. The correspondence of mRNA codon usage and the relative abundance of their cognate tRNAs is equally important for mediating the rate of polypeptide elongation. Here, we discuss recent results showing that ribosome pausing is a widely used mechanism for controlling translation and, as a result, biological transitions in health and disease

Attenuated Codon Optimality Contributes to Neural-Specific mRNA Decay in Drosophila.

Tissue-specific mRNA stability is important for cell fate and physiology, but the mechanisms involved are not fully understood. We found that zygotic mRNA stability in Drosophila correlates with codon content: optimal codons are enriched in stable transcripts associated with metabolic functions like translation, while non-optimal codons are enriched in unstable transcripts, including those associated with neural development. Bioinformatic analyses and reporter assays revealed that similar codons stabilize or destabilize mRNAs in the nervous system and other tissues, but the link between codon content and stability is attenuated in the nervous system. We confirmed that optimal codons are decoded by abundant tRNAs while non-optimal codons are decoded by less abundant tRNAs in embryos and in the nervous system. We conclude that codon optimality is a general determinant of zygotic mRNA stability, and attenuation of codon optimality allows trans-acting factors to exert greater influence over mRNA decay in the nervous system

Molecular Biology of Long Non-coding RNAs

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A quantitative assay for measuring mRNA decapping by splinted ligation reverse transcription polymerase chain reaction: qSL-RT-PCR

The degradation of messenger RNA is a critical node of gene regulation. A major pathway of mRNA decay is initiated by shortening of the poly(A) tail, followed by removal of the 5′ cap structure (decapping) and subsequent degradation. Decapping is an important determinate in the destruction of many transcripts. Detailed kinetic analysis of in vivo decapping rates is necessary to understand how this step is regulated. Importantly, the product of decapping is recalcitrant for investigation, in part due to its transient nature. As such, little in vivo kinetic information is available. Here we report the development of an assay that measures decapping of mRNAs by combining splinted ligation and quantitative RT-PCR (qSL-RT-PCR). We apply this method to determine the decapping rate constant for a natural mRNA in vivo for the first time. The qSL-RT-PCR assay may be adapted for use on any mRNA, providing a new tool to study regulation of mRNA decay

The yeast Apq12 protein affects nucleocytoplasmic mRNA transport

An important step in mRNA biogenesis is the export of mRNA from the nucleus to the cytoplasm. In this work, we provide evidence that the previously uncharacterized gene APQ12 functions in nucleocytoplasmic mRNA transport in Saccharomyces cerevisiae. First, apq12Δ strains manifest 3′ hyperadenylated mRNA similar to other previously characterized RNA export mutants. Second, bulk poly(A)(+) RNA is retained in the nucleus in apq12Δ cells. Third, an Apq12p–GFP chimeric protein is localized to the nuclear periphery. Fourth, mRNA in apq12Δ cells is stabilized, consistent with a defect in the rate of nuclear export. Interestingly, apq12Δ mutants are severely compromised for growth and display atypical cell morphology. Because this aberrant cell morphology is not seen with other viable export mutants, Apq12p must have either an additional cellular function, or preferentially impinge on the export of mRNAs regulating cell growth. Together, these findings support a role for APQ12 in nucleocytoplasmic transport of mRNA

Quantitative tRNA-sequencing uncovers metazoan tissue-specific tRNA regulation

The relative abundance of specific tRNA can impact protein production rate, folding, and messenger RNA stability. Here the authors describe QuantM-tRNA seq — a method to monitor tRNA abundance and sequence variants — and uncover distinctions in isodecoder expression between tissues that are independent of the anticodon pool of each tRNA family

The DEAD-Box Protein Dhh1 Promotes Decapping by Slowing Ribosome Movement

<div><p>Translational control and messenger RNA (mRNA) decay represent important control points in the regulation of gene expression. In yeast, the major pathway for mRNA decay is initiated by deadenylation followed by decapping and 5′–3′ exonucleolytic digestion of the mRNA. Proteins that activate decapping, such as the DEAD-box RNA helicase Dhh1, have been postulated to function by limiting translation initiation, thereby promoting a ribosome-free mRNA that is targeted for decapping. In contrast to this model, we show here that Dhh1 represses translation in vivo at a step subsequent to initiation. First, we establish that Dhh1 represses translation independent of initiation factors eIF4E and eIF3b. Second, we show association of Dhh1 on an mRNA leads to the accumulation of ribosomes on the transcript. Third, we demonstrate that endogenous Dhh1 accompanies slowly translocating polyribosomes. Lastly, Dhh1 activates decapping in response to impaired ribosome elongation. Together, these findings suggest that changes in ribosome transit rate represent a key event in the decapping and turnover of mRNA.</p> </div