Chemical Modifications to Wobble Uridines in tRNA Regulate Responses to Stress

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Abstract Cells respond to environmental stresses by regulating gene expression. Recent studies have demonstrated that stress promotes changes in the levels of enzyme-modified nucleosides found in the anticodon of many tRNAs, to regulate the translation of stress-response transcripts with specific codon usage patterns. In bacteria the wobble U34 residue of tRNA can be enzymatically modified by writers to generate thiolation, geranylation or selenation products at position 2, as well as distinct modifications to position 5, to produce 12 different modified uridines, with many predicted to play key roles in translation and stress responses. The Mnm cluster enzymes can catalyze the formation of these modified uridines and due to their uniqueness to bacteria and the importance of corresponding wobble uridines in the stress response, we propose that they can be exploited to develop technology to tag modified RNA. We have used chemical biology and structural studies to demonstrate that 2-thiouridine (s2U), geranyl-2-thiouridine (ges2U) and seleno-2-thiouridine (se2U) have different base pairing specificity. In addition, using codon analytics of all E. coli genes, we have identified codon-biased transcripts that could be translationally regulated by s2U, ges2U and se2U modifications. We have also shown that cells deficient in the wobble U modifying enzymes MnmE and MnmH are sensitive to killing by chloramphenicol (CAM) and have perturbed translation. We hypothesize that s2U, ges2U and se2U modification levels change in response to environmental stress, to regulate the translation of response proteins. In this application, we will synthesize and characterize 9 wobble modifications linked to MnmE and MnmH. Further, we propose to characterize stress-induced changes in Mnm linked tRNA modifications and determine if translation elongation of codon specific transcripts is linked to one of the 12 uridine tRNA modifications. In addition, we will develop MnmH-based molecular tools to tag and visualize thiolated tRNAs in yeast and human cells. The proposed studies are significant, as they will define a new form of translational regulation in bacteria, generate new reagents and technologies for tagging epitranscriptomic marks and will provide a unified understanding of the 12 different wobble uridines in bacterial tRNA.