Rapid and accurate translation of the genetic code into protein is fundamental to life. Yet due to lack of a suitable assay, little is known about the accuracy-determining parameters and their correlation with translational speed. Here, we develop such an assay, based on Mg 2 concentration changes, to determine maximal accuracy limits for a complete set of single-mismatch codon-anticodon interactions. We found a simple, linear trade-off between efficiency of cognate codon reading and accuracy of tRNA selection. The maximal accuracy was highest for the second codon position and lowest for the third. The results rationalize the existence of proofreading in code reading and have implications for the understanding of tRNA modifications, as well as of translation error-modulating ribosomal mutations and antibiotics. Finally, the results bridge the gap between in vivo and in vitro translation and allow us to calibrate our test tube conditions to represent the environment inside the living cell.fidelity | rate-accuracy trade-off | ribosome | protein synthesis | elongation T ranslation of the ancient and universal genetic code into protein on ribosomes requires precise mRNA decoding by aminoacyl-tRNAs (aa-tRNAs) and rapid formation of nascent peptide chains (1, 2). Codon reading by aa-tRNAs ultimately relies on the specificity of cognate in relation to noncognate codon-anticodon interactions, but two ribosome-dependent specificity enhancements greatly improve mRNA decoding (3). Firstly, bases A1492 and A1493 in the 16S rRNA of the 30S subunit have stereospecific, A-minor interactions with the first two codon-anticodon base pairs but not with the third (4-6), suggesting higher frequency of misreading of the third codon base than of the first two bases (3), in line with the wobble hypothesis (7). Secondly, the ribosome enhances the accuracy of codon reading by a twostep mechanism in which initial codon selection by a tRNA is followed by a proofreading step (8-11). That is, the ternary complex consisting of aa-tRNA, elongation factor Tu (EF-Tu), and GTP is, when cognate, activated for GTP hydrolysis on EF-Tu with high probability, whereas a noncognate ternary complex is likely to dissociate from the ribosome before GTP hydrolysis. After GTP hydrolysis on EF-Tu, a cognate aa-tRNA is selected for peptidyl transfer with high probability, whereas a noncognate aa-tRNA is likely to dissociate from the ribosome in a proofreading step before peptidyl transfer (8, 9) (Fig. 1).In spite of the central role that is played by ribosome aided reading of the genetic code in all areas of biology, very little is known about the basal parameters that provide the boundary conditions for rapid and accurate mRNA translation into protein.Most important among these parameters is the maximal possible discrimination between a cognate and a noncognate codon-anticodon interaction: the "d value." It defines the upper limit of the current single step accuracy, A, by which any aa-tRNA can separate its cognate from a specific noncognate codon. When the c...

Glomeruli are functional units in the olfactory system. The mouse olfactory bulb contains roughly 2,000 glomeruli, each receiving inputs from olfactory sensory neurons (OSNs) that express a specific odorant receptor gene. Odors typically activate many glomeruli in complex combinatorial patterns and it is unknown which features of neuronal activity in individual glomeruli contribute to odor perception. To address this, we used optogenetics to selectively activate single, genetically identified glomeruli in behaving mice. We found that mice could perceive the stimulation of a single glomerulus. Single-glomerulus stimulation was also detected on an intense odor background. In addition, different input intensities and the timing of input relative to sniffing were discriminated through one glomerulus. Our data suggest that each glomerulus can transmit odor information using identity, intensity and temporal coding cues. These multiple modes of information transmission may enable the olfactory system to efficiently identify and localize odor sources.

The mammalian main olfactory pathway detects volatile chemicals using two families of G protein-coupled receptors—a large repertoire of canonical odorant receptors (ORs) and a much smaller set of Trace Amine-Associated Receptors, or TAARs. The TAARs are evolutionarily conserved in vertebrates, including humans, suggesting an indispensible role in olfaction. However, little is known about the functional properties of TAARs when expressed in native olfactory sensory neurons. Here we describe experiments using gene targeting, electrophysiology and optical imaging to study the response properties of TAAR-expressing sensory neurons and their associated glomeruli in mice. We show that olfactory sensory neurons that express a subset of the TAAR repertoire are preferentially responsive to amines. In addition, neurons expressing one of two specific TAARs, TAAR3 and TAAR4, are highly sensitive and are also broadly tuned—responding to structurally diverse amines at high concentrations. Surprisingly, we find that TAAR4 is exquisitely sensitive, with apparent affinities for a preferred ligand, phenylethylamine, rivaling those seen with mammalian pheromone receptors. We provide evidence that this unprecedented sensitivity is mediated via receptor coupling to the canonical odorant transduction cascade. The data suggest that the TAARs are evolutionarily retained in the olfactory receptor repertoire to mediate high sensitivity detection of a biologically relevant class of odorous stimuli.

Vanadium-based materials have been extensively studied as promising cathode materials for zinc-ion batteries because of their multiple valences and adjustable ion-diffusion channels. However, the sluggish kinetics of Zn-ion intercalation and less stable layered structure remain bottlenecks that limit their further development. The present work introduces potassium ions to partially substitute ammonium ions in ammonium vanadate, leading to a subtle shrinkage of lattice distance and the increased oxygen vacancies. The resulting potassium ammonium vanadate exhibits a high discharge capacity (464 mAh g–1 at 0.1 A g–1) and excellent cycling stability (90% retention over 3000 cycles at 5 A g–1). The excellent electrochemical properties and battery performances are attributed to the rich oxygen vacancies. The introduction of K+ to partially replace NH4 + appears to alleviate the irreversible deammoniation to prevent structural collapse during ion insertion/extraction. Density functional theory calculations show that potassium ammonium vanadate has a modulated electron structure and a better zinc-ion diffusion path with a lower migration barrier.

In many species, survival depends on olfaction, yet the mechanisms that underlie olfactory sensitivity are not well understood. Here we examine how a conserved subset of olfactory receptors, the trace amine-associated receptors (TAARs), determine odor detection thresholds of mice to amines. We find that deleting all TAARs, or even single TAARs, results in significant odor detection deficits. This finding is not limited to TAARs, as the deletion of a canonical odorant receptor reduced behavioral sensitivity to its preferred ligand. Remarkably, behavioral threshold is set solely by the most sensitive receptor, with no contribution from other highly sensitive receptors. In addition, increasing the number of sensory neurons (and glomeruli) expressing a threshold-determining TAAR does not improve detection, indicating that sensitivity is not limited by the typical complement of sensory neurons. Our findings demonstrate that olfactory thresholds are set by the single highest affinity receptor and suggest that TAARs are evolutionarily conserved because they determine the sensitivity to a class of biologically relevant chemicals.