(searched for: doi:10.1146/annurev-genet-022620-101840)
Genes, Volume 12; doi:10.3390/genes12071010
The focus of this brief review is to describe the role of noncoding regulatory RNAs, including short RNAs (sRNA), transfer RNA (tRNA) fragments and microRNAs (miRNA) secreted in extracellular vesicles (EVs), in inter-kingdom communication between bacteria and mammalian (human) host cells. Bacteria secrete vesicles that contain noncoding regulatory RNAs, and recent studies have shown that the bacterial vesicles fuse with and deliver regulatory RNAs to host cells, and similar to eukaryotic miRNAs, regulatory RNAs modulate the host immune response to infection. Recent studies have also demonstrated that mammalian cells secrete EVs containing miRNAs that regulate the gut microbiome, biofilm formation and the bacterial response to antibiotics. Thus, as evidence accumulates it is becoming clear that the secretion of noncoding regulatory RNAs and miRNAs in extracellular vesicles is an important mechanism of bidirectional communication between bacteria and mammalian (human) host cells. However, additional research is necessary to elucidate how noncoding regulatory RNAs and miRNA secreted in extracellular vesicles mediate inter-kingdom communication.
Published: 3 June 2021
Science, Volume 372, pp 1057-1062; doi:10.1126/science.abg3029
It is widely hypothesized that removing cellular transfer RNAs (tRNAs)—making their cognate codons unreadable—might create a genetic firewall to viral infection and enable sense codon reassignment. However, it has been impossible to test these hypotheses. In this work, following synonymous codon compression and laboratory evolution in Escherichia coli, we deleted the tRNAs and release factor 1, which normally decode two sense codons and a stop codon; the resulting cells could not read the canonical genetic code and were completely resistant to a cocktail of viruses. We reassigned these codons to enable the efficient synthesis of proteins containing three distinct noncanonical amino acids. Notably, we demonstrate the facile reprogramming of our cells for the encoded translation of diverse noncanonical heteropolymers and macrocycles.
Frontiers in Microbiology, Volume 12; doi:10.3389/fmicb.2021.687632
In eukaryotic organisms, transfer RNA (tRNA)-derived fragments have diverse biological functions. Considering the conserved sequences of tRNAs, it is not surprising that endogenous tRNA fragments in bacteria also play important regulatory roles. Recent studies have shown that microbes secrete extracellular vesicles (EVs) containing tRNA fragments and that the EVs deliver tRNA fragments to eukaryotic hosts where they regulate gene expression. Here, we review the literature describing microbial tRNA fragment biogenesis and how the fragments secreted in microbial EVs suppress the host immune response, thereby facilitating chronic infection. Also, we discuss knowledge gaps and research challenges for understanding the pathogenic roles of microbial tRNA fragments in regulating the host response to infection.
Published: 13 May 2021
Science, Volume 372, pp 683-684; doi:10.1126/science.abi7265
When exposed to external antigens, T cells are rapidly activated to proliferate and differentiate. A genetic screen identified a mutation called elektra that causes immunodeficiency in mice through a single loss-of-function missense mutation in the Schlafen 2 (Slfn2) gene (1). Slfn2 mutation was associated with impaired T cell activation. However, whether Slfn2 regulates T cell function directly, and how, was unclear. On page 703 of this issue, Yue et al. (2) report that SLFN2 safeguards T cells from excessive stress during activation and thus facilitates the necessary up-regulation of protein translation. SLFN2 binds and shields transfer RNAs (tRNAs), essential adaptor molecules in translation of messenger RNAs (mRNAs), from stress-activated fragmentation. Without SLFN2, excessive tRNA fragmentation lowers global translation and specifically decreases the translation of key cytokine receptor proteins important for T cell activation. This study expands the role of tRNA fragmentation and implicates SLFN2 in preventing fragmentation to enable immune function.
Trends in Biochemical Sciences; doi:10.1016/j.tibs.2021.05.001
Transfer RNA (tRNA)-derived small RNAs (tsRNAs) are among the most ancient small RNAs in all domains of life and are generated by the cleavage of tRNAs. Emerging studies have begun to reveal the versatile roles of tsRNAs in fundamental biological processes, including gene silencing, ribosome biogenesis, retrotransposition, and epigenetic inheritance, which are rooted in tsRNA sequence conservation, RNA modifications, and protein-binding abilities. We summarize the mechanisms of tsRNA biogenesis and the impact of RNA modifications, and propose how thinking of tsRNA functionality from an evolutionary perspective urges the expansion of tsRNA research into a wider spectrum, including cross-tissue/cross-species regulation and harnessing of the 'tsRNA code' for precision medicine.
Nature, Volume 23, pp 424-436; doi:10.1038/s41556-021-00652-7
Although high-throughput RNA sequencing (RNA-seq) has greatly advanced small non-coding RNA (sncRNA) discovery, the currently widely used complementary DNA library construction protocol generates biased sequencing results. This is partially due to RNA modifications that interfere with adapter ligation and reverse transcription processes, which prevent the detection of sncRNAs bearing these modifications. Here, we present PANDORA-seq (panoramic RNA display by overcoming RNA modification aborted sequencing), employing a combinatorial enzymatic treatment to remove key RNA modifications that block adapter ligation and reverse transcription. PANDORA-seq identified abundant modified sncRNAs—mostly transfer RNA-derived small RNAs (tsRNAs) and ribosomal RNA-derived small RNAs (rsRNAs)—that were previously undetected, exhibiting tissue-specific expression across mouse brain, liver, spleen and sperm, as well as cell-specific expression across embryonic stem cells (ESCs) and HeLa cells. Using PANDORA-seq, we revealed unprecedented landscapes of microRNA, tsRNA and rsRNA dynamics during the generation of induced pluripotent stem cells. Importantly, tsRNAs and rsRNAs that are downregulated during somatic cell reprogramming impact cellular translation in ESCs, suggesting a role in lineage differentiation. Shi et al. profiled small non-coding RNAs (sncRNAs) through PANDORA-seq, which identified tissue-specific transfer RNA- and ribosomal RNA-derived small RNAs, as well as sncRNAs, with dynamic changes during induced pluripotent stem cell reprogramming.
Frontiers in Molecular Biosciences, Volume 8; doi:10.3389/fmolb.2021.643701
Cellular tRNAs appear today as a diverse population of informative macromolecules with conserved general elements ensuring essential common functions and different and distinctive features securing specific interactions and activities. Their differential expression and the variety of post-transcriptional modifications they are subject to, lead to the existence of complex repertoires of tRNA populations adjusted to defined cellular states. Despite the tRNA-coding genes redundancy in prokaryote and eukaryote genomes, it is surprising to note the absence of genes coding specific translational-active isoacceptors throughout the phylogeny. Through the analysis of different releases of tRNA databases, this review aims to provide a general summary about those “missing tRNA genes.” This absence refers to both tRNAs that are not encoded in the genome, as well as others that show critical sequence variations that would prevent their activity as canonical translation adaptor molecules. Notably, while a group of genes are universally missing, others are absent in particular kingdoms. Functional information available allows to hypothesize that the exclusion of isodecoding molecules would be linked to: 1) reduce ambiguities of signals that define the specificity of the interactions in which the tRNAs are involved; 2) ensure the adaptation of the translational apparatus to the cellular state; 3) divert particular tRNA variants from ribosomal protein synthesis to other cellular functions. This leads to consider the “missing tRNA genes” as a source of putative non-canonical tRNA functions and to broaden the concept of adapter molecules in ribosomal-dependent protein synthesis.
Frontiers in Microbiology, Volume 12; doi:10.3389/fmicb.2021.634004
Transfer RNAs (tRNAs) are widely known for their roles in the decoding of the linear mRNA information into amino acid sequences of proteins. They are also multifunctional platforms in the translation process and have other roles beyond translation, including sensing amino acid abundance, interacting with the general stress response machinery, and modulating cellular adaptation, survival, and death. In this mini-review, we focus on the emerging role of tRNA genes in the organization and modification of the genomic architecture of yeast and the role of tRNA misexpression and decoding infidelity in genome stability, evolution, and adaption. We discuss published work showing how quickly tRNA genes can mutate to meet novel translational demands, how tRNAs speed up genome evolution, and how tRNA genes can be sites of genomic instability. We highlight recent works showing that loss of tRNA decoding fidelity and small alterations in tRNA expression have unexpected and profound impacts on genome stability. By dissecting these recent evidence, we hope to lay the groundwork that prompts future investigations on the mechanistic interplay between tRNAs and genome modification that likely triggers genome evolution.