Artificial Riboswitches: Synthetic mRNA‐Based Regulators of Gene Expression

For half a century, bacterial regulation of gene expression has been known to be dominated by proteins that interact with metabolites, which results in altered transcription initiation. lthough the expression of the majority of genes is controlled by protein-based mechanisms, the discovery of RNA-based feedback devices that enable regulation of expression without the need for engaged proteins came as a surprise. Breaker and co-workers initially discovered that the use of such mechanisms, termed riboswitches, is widespread in bacteria. For excellent reviews that highlight naturally occurring riboswitches, we refer to the recent literature. Riboswitches are typically located in the 5’-untranslated region (5’-UTR) of bacterial mRNA, and consist mainly of a first domain (called aptamer domain) that specifically senses a metabolite, and a second domain (the expression platform) that facilitates control over transcription termination or translation initiation by a structural rearrangement (see Scheme 1). With respect to the revolutionary findings of Breaker and coworkers, it is very intriguing that researchers have successfully constructed similar, artificial systems even several years before naturally occurring riboswitches were discovered. The generation of such man-made, RNA-based regulators was possible by using aptamer technology for the recognition of ligands by RNAs. Aptamers are in-vitro-selected nucleic acid sequences that specifically bind to a ligand of choice. Such artificial, RNA-based switches enable the control of gene expression, uncoupled from the intrinsic metabolism. Although natural riboACHTUNGTRENNUNGswitches are mainly found in bacteria, artificial systems have been constructed for eukaryotic organisms as well. Such tailormade regulatory devices should prove of value as tools in biotechnology as well as synthetic biology applications. Here, we give an overview of the different concepts that are based on the insertion of ligand-sensing elements into mRNAs, thereby enabling the regulation of expression of the respective message. Due to space restrictions we will neither discuss artificial trans-acting mechanisms such as small-molecule-regulated, RNA-based transcriptional activators, nor ligand-controlled antisense constructs for the regulation of gene expression.

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