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1. A Decrease in Serine Levels during Growth Transition Triggers Biofilm Formation in Bacillus subtilis

   https://doi.org/10.1128/JB.00155-19  

   Greenwich, Jennifer; Reverdy, Alicyn; Gozzi, Kevin; Di Cecco, Grace; Tashjian, Tommy; Godoy-Carter, Veronica; Chai, Yunrong; Henkin, Tina M. (August 2019, Journal of Bacteriology)

   ABSTRACT Biofilm development in Bacillus subtilis is regulated at multiple levels. While a number of known signals that trigger biofilm formation do so through the activation of one or more sensory histidine kinases, it was discovered that biofilm activation is also coordinated by sensing intracellular metabolic signals, including serine starvation. Serine starvation causes ribosomes to pause on specific serine codons, leading to a decrease in the translation rate of sinR , which encodes a master repressor for biofilm matrix genes and ultimately triggers biofilm induction. How serine levels change in different growth stages, how B. subtilis regulates intracellular serine levels, and how serine starvation triggers ribosomes to pause on selective serine codons remain unknown. Here, we show that serine levels decrease as cells enter stationary phase and that unlike most other amino acid biosynthesis genes, expression of serine biosynthesis genes decreases upon the transition into stationary phase. The deletion of the gene for a serine deaminase responsible for converting serine to pyruvate led to a delay in biofilm formation, further supporting the idea that serine levels are a critical intracellular signal for biofilm activation. Finally, we show that levels of all five serine tRNA isoacceptors are decreased in stationary phase compared with exponential phase. However, the three isoacceptors recognizing UCN serine codons are reduced to a much greater extent than the two that recognize AGC and AGU serine codons. Our findings provide evidence for a link between serine homeostasis and biofilm development in B. subtilis . IMPORTANCE In Bacillus subtilis , biofilm formation is triggered in response to environmental and cellular signals. It was proposed that serine limitation acts as a proxy for nutrient status and triggers biofilm formation at the onset of biofilm entry through a novel signaling mechanism caused by global ribosome pausing on selective serine codons. In this study, we reveal that serine levels decrease at the biofilm entry due to catabolite control and a serine shunt mechanism. We also show that levels of five serine tRNA isoacceptors are differentially decreased in stationary phase compared with exponential phase; three isoacceptors recognizing UCN serine codons are reduced much more than the two recognizing AGC and AGU codons. This finding indicates a possible mechanism for selective ribosome pausing. 

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2. Alanyl-tRNA Synthetase Quality Control Prevents Global Dysregulation of the Escherichia coli Proteome

   https://doi.org/10.1128/mBio.02921-19  

   Kelly, Paul; Backes, Nicholas; Mohler, Kyle; Buser, Christopher; Kavoor, Arundhati; Rinehart, Jesse; Phillips, Gregory; Ibba, Michael; Merrikh, Houra (December 2019, mBio)

   ABSTRACT Mechanisms have evolved to prevent errors in replication, transcription, and translation of genetic material, with translational errors occurring most frequently. Errors in protein synthesis can occur at two steps, during tRNA aminoacylation and ribosome decoding. Recent advances in protein mass spectrometry have indicated that previous reports of translational errors have potentially underestimated the frequency of these events, but also that the majority of translational errors occur during ribosomal decoding, suggesting that aminoacylation errors are evolutionarily less tolerated. Despite that interpretation, there is evidence that some aminoacylation errors may be regulated, and thus provide a benefit to the cell, while others are clearly detrimental. Here, we show that while it has been suggested that regulated Thr-to-Ser substitutions may be beneficial, there is a threshold beyond which these errors are detrimental. In contrast, we show that errors mediated by alanyl-tRNA synthetase (AlaRS) are not well tolerated and induce a global stress response that leads to gross perturbation of the Escherichia coli proteome, with potentially catastrophic effects on fitness and viability. Tolerance for Ala mistranslation appears to be much lower than with other translational errors, consistent with previous reports of multiple proofreading mechanisms targeting mischarged tRNA Ala . These results demonstrate the essential role of aminoacyl-tRNA proofreading in optimizing cellular fitness and suggest that any potentially beneficial effects of mistranslation may be confined to specific amino acid substitutions. IMPORTANCE Errors in protein synthesis have historically been assumed to be detrimental to the cell. While there are many reports that translational errors are consequential, there is a growing body of evidence that some mistranslation events may be tolerated or even beneficial. Using two models of mistranslation, we compare the direct phenotypic effects of these events in Escherichia coli . This work provides insight into the threshold for tolerance of specific mistranslation events that were previously predicted to be broadly neutral to proteome integrity. Furthermore, these data reveal the effects of mistranslation beyond the general unfolded stress response, leading to global translational reprogramming. 

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3. Oxidation of phenylalanyl-tRNA synthetase positively regulates translational quality control

   https://doi.org/10.1073/pnas.1901634116  

   Steiner, Rebecca E.; Kyle, Amanda M.; Ibba, Michael (May 2019, Proceedings of the National Academy of Sciences)

   Accurate translation of the genetic code is maintained in part by aminoacyl-tRNA synthetases (aaRS) proofreading mechanisms that ensure correct attachment of a cognate amino acid to a transfer RNA (tRNA). During environmental stress, such as oxidative stress, demands on aaRS proofreading are altered by changes in the availability of cytoplasmic amino acids. For example, oxidative stress increases levels of cytotoxic tyrosine isomers, noncognate amino acids normally excluded from translation by the proofreading activity of phenylalanyl-tRNA synthetase (PheRS). Here we show that oxidation of PheRS induces a conformational change, generating a partially unstructured protein. This conformational change does not affect Phe or Tyr activation or the aminoacylation activity of PheRS. However, in vitro and ex vivo analyses reveal that proofreading activity to hydrolyze Tyr-tRNA Phe is increased during oxidative stress, while the cognate Phe-tRNA Phe aminoacylation activity is unchanged. In HPX − , Escherichia coli that lack reactive oxygen-scavenging enzymes and accumulate intracellular H 2 O 2 , we found that PheRS proofreading is increased by 11%, thereby providing potential protection against hazardous cytoplasmic m -Tyr accumulation. These findings show that in response to oxidative stress, PheRS proofreading is positively regulated without negative effects on the enzyme’s housekeeping activity in translation. Our findings also illustrate that while the loss of quality control and mistranslation may be beneficial under some conditions, increased proofreading provides a mechanism for the cell to appropriately respond to environmental changes during oxidative stress. 

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4. Systematic identification of tRNA genes in Drosophila melanogaster

   Marygold, SJ; Chan, PP; Lowe, TM (May 2022, microPublication biology)

   Transfer RNAs (tRNAs) are ubiquitous adapter molecules that link specific codons in messenger RNA (mRNA) with their corresponding amino acids during protein synthesis. The tRNA genes of Drosophila have been investigated for over half a century but have lacked systematic identification and nomenclature. Here, we review and integrate data within FlyBase and the Genomic tRNA Database (GtRNAdb) to identify the full complement of tRNA genes in the D. melanogaster nuclear and mitochondrial genomes. We apply a logical and informative nomenclature to all tRNA genes, and provide an overview of their characteristics and genomic features. 

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5. Truncation-Free Genetic Code Expansion with Tetrazine Amino Acids for Quantitative Protein Ligations

   https://doi.org/10.1021/acs.bioconjchem.3c00380  

   Eddins, Alex J; Bednar, Riley M; Jana, Subhashis; Pung, Abigail H; Mbengi, Lea; Meyer, Kyle; Perona, John J; Cooley, Richard B; Karplus, P Andrew; Mehl, Ryan A (December 2023, Bioconjugate Chemistry)

   Quantitative labeling of biomolecules is necessary to advance areas of antibody–drug conjugation, super-resolution microscopy imaging of molecules in live cells, and determination of the stoichiometry of protein complexes. Bio-orthogonal labeling to genetically encodable noncanonical amino acids (ncAAs) offers an elegant solution; however, their suboptimal reactivity and stability hinder the utility of this method. Previously, we showed that encoding stable 1,2,4,5-tetrazine (Tet)-containing ncAAs enables rapid, complete conjugation, yet some expression conditions greatly limited the quantitative reactivity of the Tet-protein. Here, we demonstrate that reduction of on-protein Tet ncAAs impacts their reactivity, while the leading cause of the unreactive protein is near-cognate suppression (NCS) of UAG codons by endogenous aminoacylated tRNAs. To overcome incomplete conjugation due to NCS, we developed a more catalytically efficient tRNA synthetase and developed a series of new machinery plasmids harboring the aminoacyl tRNA synthetase/tRNA pair (aaRS/tRNA pair). These plasmids enable robust production of homogeneously reactive Tet-protein in truncation-free cell lines, eliminating the contamination caused by NCS and protein truncation. Furthermore, these plasmid systems utilize orthogonal synthetic origins, which render these machinery vectors compatible with any common expression system. Through developing these new machinery plasmids, we established that the aaRS/tRNA pair plasmid copy-number greatly affects the yields and quality of the protein produced. We then produced quantitatively reactive soluble Tet-Fabs, demonstrating the utility of this system for rapid, homogeneous conjugations of biomedically relevant proteins. 

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