Riboswitches are common gene regulatory units mostly found in bacteria that are capable of altering gene expression in response to a small molecule. These structured RNA elements consist of two modular subunits: an aptamer domain that binds with high specificity and affinity to a target ligand and an expression platform that transduces ligand binding to a gene expression output. Significant progress has been made in engineering novel aptamer domains for new small molecule inducers of gene expression. Modified expression platforms have also been optimized to function when fused with both natural and synthetic aptamer domains. As this field expands, the use of these privileged scaffolds has permitted the development of tools such as RNA-based fluorescent biosensors. In this review, we summarize the methods that have been developed to engineer new riboswitches and highlight applications of natural and synthetic riboswitches in enzyme and strain engineering, in controlling gene expression and cellular physiology, and in real-time imaging of cellular metabolites and signals.

A synthetic substrate enables a new colorimetric screen for terpene synthase cyclization activity, facilitating the engineering of these enzymes. Using directed evolution, the thermostability of the sesquiterpene synthase BcBOT2 was increased without the loss of other properties. The technique also enabled rapid optimization of conditions for expression and stabilization in lysate of another terpene synthase, SSCG_02150.

A variant of P450 from Bacillus megaterium five mutations away from wild type is a highly active catalyst for cyclopropanation of a variety of acrylamide and acrylate olefins with ethyl diazoacetate (EDA). The very high rate of reaction enabled by histidine ligation allowed the reaction to be conducted under aerobic conditions. The promiscuity of this catalyst for a variety of substrates containing amides has enabled synthesis of a small library of precursors to levomilnacipran derivatives.

A synthetic substrate enables a new colorimetric screen for terpene synthase cyclization activity, facilitating the engineering of these enzymes. Using directed evolution, the thermostability of the sesquiterpene synthase BcBOT2 was increased without the loss of other properties. The technique also enabled rapid optimization of conditions for expression and stabilization in lysate of another terpene synthase, SSCG_02150.Keywords directed evolution; high-throughput screening; protein engineering; terpenes; thermostability The development of high-throughput assays can be extremely challenging, yet is essential for many applications in drug discovery and enzyme engineering. [1] Directed evolution has proven to be a reliable method for optimizing the performance of enzymes in a variety of applications, [2] but it requires an appropriate high throughput assay for screening mutant libraries. Terpene synthases catalyze the key cyclization step in the biosynthesis of terpenoids, which are by far the largest class of natural compound and are highly valued as medicines, materials, fuels, and chemicals. [3] Although metabolic engineering efforts have improved access to some terpenoids by production in microbial hosts, the terpene synthase enzymes responsible for the cyclization of linear isoprenoid diphosphates into cyclic terpenoids have proven difficult to engineer. [4] The cyclizations proceed through complex carbocation bond-forming reactions and migrations, and are terminated by either elimination or aqueous quenching of carbocationic intermediates. The complexity of these processes

Mutation of the sesquiterpene synthase Cop2 was conducted with a high-throughput screen for the cyclization activity using a non-natural substrate. A mutant of Cop2 was identified that contained three amino acid substitutions. This mutant, 17H2, converted the natural substrate FPP into germacrene D-4-ol with 77% selectivity. This selectivity is in contrast to that of the parent enzyme in which germacrene D-4-ol is produced as 29% and α-cadinol is produced as 46% of the product mixture. The mutations were shown to each contribute to this selectivity, and a homology model suggested that the mutations lie near to the active site though would be unlikely to be targeted for mutation by rational methods. Kinetic comparisons show that 17H2 maintains a k cat /K M of 0.62 mM −1 s −1 , which is nearly identical to that of the parent Cop2, which had a k cat /K M of 0.58 mM −1 s −1 .