Over one hundred types of chemical modifications have been identified in cellular RNAs. While the 5’ cap modification and the poly(A) tail of eukaryotic messenger RNA play key roles in regulation, internal modifications are gaining attention for their roles in mRNA metabolism. The most abundant internal modification is N6-methyladenosine (m6A), and identification of proteins that install, recognize, and remove this and other marks have revealed roles for mRNA modification in nearly every aspect of the mRNA lifecycle, as well as in various cellular, developmental, and disease processes. Abundant noncoding RNAs such as transfer RNAs, ribosomal RNAs and spliceosomal RNAs are also heavily modified and depend on the modifications for their biogenesis and function. Our understanding of the biological contributions of these different chemical modifications is beginning to take shape, but it’s clear that in both coding and noncoding RNA, dynamic modifications represent a new layer of control of genetic information.

Spinach is an in vitro selected RNA aptamer that binds a GFP-like ligand and activates its green fluorescence.Spinach is thus an RNA analog of GFP, and has potentially widespread applications for in vivo labeling and imaging. We used antibody-assisted crystallography to determine the structures of Spinach both with and without bound fluorophore at 2.2 and 2.4 Å resolution, respectively. Spinach RNA has an elongated structure containing two helical domains separated by an internal bulge that folds into a G-quadruplex motif of unusual topology. The G-quadruplex motif and adjacent nucleotides comprise a partially pre-formed binding site for the fluorophore.The fluorophore binds in a planar conformation and makes extensive aromatic stacking and hydrogen bond interactions with the RNA. Our findings provide a foundation for structure-based engineering of new fluorophore-binding RNA aptamers.

Summary Transfer RNA (tRNA) is a central component of protein synthesis and cell signaling network. One salient feature of tRNA is its heavily modified status, which can critically impact its function. Here we show that mammalian ALKBH1 is a tRNA demethylase. It mediates the demethylation of N1-methyladenosine (m1A) in tRNAs. The ALKBH1-catalyzed demethylation of the target tRNAs results in attenuated translation initiation and their decreased usage in protein synthesis. This process is dynamic and responds to glucose availability to affect translation. Our results uncover reversible methylation of tRNA as a new regulatory mechanism of post-transcriptional gene expression.

Eukaryotic transfer RNAs contain on average 14 modifications. Investigations of their biological functions require the determination of the modification sites and the dynamic variations of the modification fraction. Base methylation represents a major class of tRNA modification. Although many approaches have been used to identify tRNA base methylations, including sequencing, they are generally qualitative and do not report the information on the modification fraction. Dynamic mRNA modifications have been shown to play important biological roles; yet, the extent of tRNA modification fractions has not been reported systemically. Here we take advantage of a recently developed high-throughput sequencing method (DM-tRNA-seq) to identify and quantify tRNA base methylations located at the Watson-Crick face in HEK293T cells at single base resolution. We apply information derived from both base mutations and positional stops from sequencing using a combination of demethylase treatment and cDNA synthesis by a thermophilic reverse transcriptase to compile a quantitative "Modification Index" (MI) for six base methylations in human tRNA and rRNA. MI combines the metrics for mutational and stop components from alignment of sequencing data without demethylase treatment, and the modifications are validated in the sequencing data upon demethylase treatment. We identify many new methylation sites in both human nuclear and mitochondrial-encoded tRNAs not present in the RNA modification databases. The potentially quantitative nature of the MI values obtained from sequencing is validated by primer extension of several tRNAs. Our approach should be widely applicable to identify tRNA methylation sites, analyze comparative fractional modifications, and evaluate the modification dynamics between different samples.

SummaryTransfer RNA (tRNA) is a central component of protein synthesis and cell signaling network. One salient feature of tRNA is its heavily modified status, which can critically impact its function. Here we show that mammalian ALKBH1 is a tRNA demethylase. It mediates the demethylation of N 1 -methyladenosine (m 1 A) in tRNAs. The ALKBH1-catalyzed demethylation of the target tRNAs results in attenuated translation initiation and their decreased usage in protein synthesis. This process is dynamic and responds to glucose availability to affect translation. Our results uncover reversible methylation of tRNA as a new regulatory mechanism of post-transcriptional gene expression. In briefReversible tRNA methylation helps translation respond to nutrient availability.

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No abstract

Transfer RNA (tRNA) decodes mRNA codons when aminoacylated (charged) with an amino acid at its 3′ end. Charged tRNAs turn over rapidly in cells, and variations in charged tRNA fractions are known to be a useful parameter in cellular responses to stress. tRNA charging fractions can be measured for individual tRNA species using acid denaturing gels, or comparatively at the genome level using microarrays. These hybridization-based approaches cannot be used for high resolution analysis of mammalian tRNAs due to their large sequence diversity. Here we develop a high-throughput sequencing method that enables accurate determination of charged tRNA fractions at single-base resolution (Charged DM-tRNA-seq). Our method takes advantage of the recently developed DM-tRNA-seq method, but includes additional chemical steps that specifically remove the 3′A residue in uncharged tRNA. Charging fraction is obtained by counting the fraction of A-ending reads versus A+C-ending reads for each tRNA species in the same sequencing reaction. In HEK293T cells, most cytosolic tRNAs are charged at >80% levels, whereas tRNASer and tRNAThr are charged at lower levels. These low charging levels were validated using acid denaturing gels. Our method should be widely applicable for investigations of tRNA charging as a parameter in biological regulation.