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The use of chemical exchange saturation transfer NMR reveals a previously hidden excited conformational state of the fluoride riboswitch, providing a model in which ligand binding allosterically suppresses a linchpin base pair to activate transcription. Riboswitches control gene expression through ligand-dependent structural rearrangements of the sensing aptamer domain. However, we found that the Bacillus cereus fluoride riboswitch aptamer adopts identical tertiary structures in solution with and without ligand. Using chemical-exchange saturation transfer (CEST) NMR spectroscopy, we revealed that the structured ligand-free aptamer transiently accesses a low-populated (∼1%) and short-lived (∼3 ms) excited conformational state that unravels a conserved 'linchpin' base pair to signal transcription termination. Upon fluoride binding, this highly localized, fleeting process is allosterically suppressed, which activates transcription. We demonstrated that this mechanism confers effective fluoride-dependent gene activation over a wide range of transcription rates, which is essential for robust toxicity responses across diverse cellular conditions. These results unveil a novel switching mechanism that employs ligand-dependent suppression of an aptamer excited state to coordinate regulatory conformational transitions rather than adopting distinct aptamer ground-state tertiary architectures, exemplifying a new mode of ligand-dependent RNA regulation.

Metal-bound peptides can catalyze simple reactions such as ester hydrolysis and may have been the starting point for the evolution of modern enzymes. Studer et al. selected progressively more-proficient variants of a small protein derived from a computationally designed zinc-binding peptide. The resulting enzyme could perform the trained reaction at rates typical for naturally evolved enzymes and serendipitously developed a strong preference for a single enantiomer of the substrate. A structure of the final catalyst highlights how small, progressive changes can remodel both catalytic residues and protein architecture in unpredictable ways. Science , this issue p. 1285 A metal-binding peptide becomes a potent catalyst for ester hydrolysis. Primordial sequence signatures in modern proteins imply ancestral origins tracing back to simple peptides. Although short peptides seldom adopt unique folds, metal ions might have templated their assembly into higher-order structures in early evolution and imparted useful chemical reactivity. Recapitulating such a biogenetic scenario, we have combined design and laboratory evolution to transform a zinc-binding peptide into a globular enzyme capable of accelerating ester cleavage with exacting enantiospecificity and high catalytic efficiency ( k cat / K M ~ 10 6 M −1 s −1 ). The simultaneous optimization of structure and function in a naïve peptide scaffold not only illustrates a plausible enzyme evolutionary pathway from the distant past to the present but also proffers exciting future opportunities for enzyme design and engineering.

Recent breeding efforts in Brassica have focused on the development of new oilseed feedstock crop for biofuels (e.g., ethanol, biodiesel, bio-jet fuel), bio-industrial uses (e.g., bio-plastics, lubricants), specialty fatty acids (e.g., erucic acid), and producing low glucosinolates levels for oilseed and feed meal production for animal consumption. We identified a novel opportunity to enhance the availability of nutritious, fresh leafy greens for human consumption. Here, we demonstrated the efficacy of disarming the ‘mustard bomb’ reaction in reducing pungency upon the mastication of fresh tissue—a major source of unpleasant flavor and/or odor in leafy Brassica. Using gene-specific mutagenesis via CRISPR-Cas12a, we created knockouts of all functional copies of the type-I myrosinase multigene family in tetraploid Brassica juncea. Our greenhouse and field trials demonstrate, via sensory and biochemical analyses, a stable reduction in pungency in edited plants across multiple environments. Collectively, these efforts provide a compelling path toward boosting the human consumption of nutrient-dense, fresh, leafy green vegetables.

The rise of antibiotic resistance calls for new therapeutics targeting resistance factors such as the New Delhi metallo-β-lactamase 1 (NDM-1), a bacterial enzyme that degrades β-lactam antibiotics. We present structure-guided computational methods for designing peptide macrocycles built from mixtures of L- and D-amino acids that are able to bind to and inhibit targets of therapeutic interest. Our methods explicitly consider the propensity of a peptide to favor a binding-competent conformation, which we found to predict rank order of experimentally observed IC50 values across seven designed NDM-1- inhibiting peptides. We were able to determine X-ray crystal structures of three of the designed inhibitors in complex with NDM-1, and in all three the conformation of the peptide is very close to the computationally designed model. In two of the three structures, the binding mode with NDM-1 is also very similar to the design model, while in the third, we observed an alternative binding mode likely arising from internal symmetry in the shape of the design combined with flexibility of the target. Although challenges remain in robustly predicting target backbone changes, binding mode, and the effects of mutations on binding affinity, our methods for designing ordered, binding-competent macrocycles should have broad applicability to a wide range of therapeutic targets.

Approximately one third of proteins are thought to bind metal ions. These ions often improve the protein's structural stability or impart functions such as conformational changes or catalytic activity. Due to their prevalence and functional roles, metal binding sites are attractive targets for protein design. Zinc binding sites are among the most common design targets due to their well-defined geometry and wide range of functions. Previous studies have successfully designed zinc binding sites by mutating residues in native proteins or by designing symmetric de novo scaffolds with zinc-coordinating residues; however, achieving atomic-level accuracy in modelling these sites remains a challenge. Here, we explore possible reasons for this continued challenge and introduce novel approaches for metalloprotein design. We first examine two crystal structures of metalloproteins designed by introducing coordinating residues into native scaffolds. We find that one of these sites shows significant changes in backbone conformation versus the predicted structure to accommodate the formation of second-shell hydrogen bonds, and the second binding site fails to form in the absence of such interactions. When these interactions are explicitly modelled, we find evidence of zinc binding at the modeled site. We next describe the development of a novel method for metalloprotein design based on the SEWING method for de novo backbone formation. This method, which we call Ligand SEWING, begins with a partially-coordinated metal binding site from a native protein and appends structural motifs to complete metal coordination and extend the protein backbone. Using this method, we design four proteins that are monomeric and bind zinc with high nanomolar to low micromolar affinity. During the design process, we note that designs are more successful when they do not model the charge of the zinc ion, indicating deficits in modelling binding site electrostatics (most likely due to not distributing the charge over coordinating residues). While accurate modelling of these interactions will require scoring modifications and possibly improved computational power, they may be partially circumvented by ignoring the ion charge and manually modelling second-shell interactions. Alternatively, high-throughput design and screening methods may allow functional design without atomic-level accuracy in modeling.