Under stress, such as heat shock, the N6-methyladenosine (m6A) modification is shown to accumulate primarily in the 5′ untranslated region of induced mRNAs owing to the translocation of an m6A interacting protein, YTHDF2, into the nucleus, resulting in increased cap-independent translation of these mRNAs, indicating one possible mechanism by which stress-responsive genes can be preferentially expressed. The distribution of the N6-methyladenosine (m6A) base modification along mRNAs is uneven — it is under-represented in the 5′ untranslated region (5′ UTR) compared to other regions. Shu-Bing Qian and colleagues have found that under a stress such as heat shock, the m6A modification accumulates primarily in the 5′ UTR of induced mRNAs due to the translocation of an m6A interacting protein, YTHDF2, into the nucleus. YTHDF2 blocks the action of the m6A 'eraser' protein FTO, resulting in increased cap-independent translation of these mRNAs. The results indicate one possible mechanism by which stress-responsive genes can be preferentially expressed. The most abundant mRNA post-transcriptional modification is N6-methyladenosine (m6A), which has broad roles in RNA biology1,2,3,4,5. In mammalian cells, the asymmetric distribution of m6A along mRNAs results in relatively less methylation in the 5′ untranslated region (5′UTR) compared to other regions6,7. However, whether and how 5′UTR methylation is regulated is poorly understood. Despite the crucial role of the 5′UTR in translation initiation, very little is known about whether m6A modification influences mRNA translation. Here we show that in response to heat shock stress, certain adenosines within the 5′UTR of newly transcribed mRNAs are preferentially methylated. We find that the dynamic 5′UTR methylation is a result of stress-induced nuclear localization of YTHDF2, a well-characterized m6A ‘reader’. Upon heat shock stress, the nuclear YTHDF2 preserves 5′UTR methylation of stress-induced transcripts by limiting the m6A ‘eraser’ FTO from demethylation. Remarkably, the increased 5′UTR methylation in the form of m6A promotes cap-independent translation initiation, providing a mechanism for selective mRNA translation under heat shock stress. Using Hsp70 mRNA as an example, we demonstrate that a single m6A modification site in the 5′UTR enables translation initiation independent of the 5′ end N7-methylguanosine cap. The elucidation of the dynamic features of 5′UTR methylation and its critical role in cap-independent translation not only expands the breadth of physiological roles of m6A, but also uncovers a previously unappreciated translational control mechanism in heat shock response.

MicroRNAs (miRNAs) are RNA sequences of ∼22 nucleotides that mediate post-transcriptional regulation of specific mRNAs. miRNA sequences are dispersed throughout the genome and are classified as intergenic (between genes) or intronic (embedded into a gene). Intergenic miRNAs are expressed by their own promoter, and until recently, it was supposed that intronic miRNAs are transcribed from their host gene. Here, we performed a genomic analysis of currently known intronic miRNA regions and observed that ∼35% of intronic miRNAs have upstream regulatory elements consistent with promoter function. Among all intronic miRNAs, 30% have associated Pol II regulatory elements, including transcription start sites, CpG islands, expression sequence tags, and conserved transcription factor binding sites, while 5% contain RNA Pol III regulatory elements (A/B box sequences). We cloned intronic regions encompassing miRNAs and their upstream Pol II (miR-107, miR-126, miR-208b, miR-548f-2, miR-569, and miR-590) or Pol III (miR-566 and miR-128-2) sequences into a promoterless plasmid, and confirmed that miRNA expression occurs independent of host gene transcription. For miR-128-2, a miRNA overexpressed in acute lymphoblastic leukemia, ChIP analysis suggests dual regulation by both intronic (Pol III) and host gene (Pol II) promoters. These data support complex regulation of intronic miRNA expression, and have relevance to disregulation in disease settings.

QTI-seq (quantitative translation initiation sequencing) maps the position of the start codon and thus allows the determination of initiation efficiency in response to various stimuli, such as starvation, in cell culture and in vivo. Cells have evolved exquisite mechanisms to fine-tune the rate of protein synthesis in response to stress. Systemic mapping of start-codon positions and precise measurement of the corresponding initiation rate would transform our understanding of translational control. Here we present quantitative translation initiation sequencing (QTI-seq), with which the initiating ribosomes can be profiled in real time at single-nucleotide resolution. Resultant initiation maps not only delineated variations of start-codon selection but also highlighted a dynamic range of initiation rates in response to nutrient starvation. The integrated data set provided unique insights into principles of alternative translation and mechanisms controlling different aspects of translation initiation. With RiboTag mice, QTI-seq permitted tissue-specific profiling of initiating ribosomes in vivo. Liver cell–specific ribosome profiling uncovered a robust translational reprogramming of the proteasome system in fasted mice. Our findings illuminated the prevalence and dynamic nature of translational regulation pivotal to physiological adaptation in vivo.

Cleavage stimulation factor 64 kDa (CstF64) is an essential pre-mRNA 3′ processing factor and an important regulator of alternative polyadenylation (APA). Here we characterized CstF64–RNA interactions in vivo at the transcriptome level and investigated the role of CstF64 in global APA regulation through individual nucleotide resolution UV crosslinking and immunoprecipitation sequencing and direct RNA sequencing analyses. We observed highly specific CstF64–RNA interactions at poly(A) sites (PASs), and we provide evidence that such interactions are widely variable in affinity and may be differentially required for PAS recognition. Depletion of CstF64 by RNAi has a relatively small effect on the global APA profile, but codepletion of the CstF64 paralog CstF64τ leads to greater APA changes, most of which are characterized by the increased relative use of distal PASs. Finally, we found that CstF64 binds to thousands of dormant intronic PASs that are suppressed, at least in part, by U1 small nuclear ribonucleoproteins. Taken together, our findings provide insight into the mechanisms of PAS recognition and identify CstF64 as an important global regulator of APA.