An inhibitory RNA aptamer against the lambda cI repressor shows transcriptional activator activity in vivo

A Novel RNA Aptamer as Synthetic Inducer of DasR Controlled Transcription

Progress in the synthetic biology field is driven by the development of new tools for synthetic circuit engineering. Traditionally, the focus has relied on protein-based designs. In recent years, the use of RNA-based tools has tremendously increased, due to their versatile functionality and applicability. A promising class of molecules is RNA aptamers, small, single-stranded RNA molecules that bind to a target molecule with high affinity and specificity. When targeting bacterial repressors, RNA aptamers allow one to add a new layer to an established protein-based regulation. In the present study, we selected an RNA aptamer binding the bacterial repressor DasR, preventing its binding to its operator sequence and activating DasR-controlled transcription in vivo. This was made possible only by the combination of an in vitro selection and subsequent in vivo screening. Next-generation sequencing of the selection process proved the importance of the in vivo screening for the discovery of aptamers functioning in the cell. Mutational and biochemical studies led to the identification of the minimal necessary binding motif. Taken together, the resulting combination of bacterial repressor and RNA aptamer enlarges the synthetic biology toolbox by adding a new level of regulation.

Aptamers: Uptake mechanisms and intracellular applications

The structural flexibility and small size of aptamers enable precise recognition of cellular elements for imaging and therapeutic applications. The process by which aptamers are taken into cells depends on their targets but is typically clathrin-mediated endocytosis or macropinocytosis. After internalization, most aptamers are transported to endosomes, lysosomes, endoplasmic reticulum, Golgi apparatus, and occasionally mitochondria and autophagosomes. Intracellular aptamers, or “intramers,” have versatile functions ranging from intracellular RNA imaging, gene regulation, and therapeutics to allosteric modulation, which we discuss in this review. Immune responses to therapeutic aptamers and the effects of G-quadruplex structure on aptamer function are also discussed.

Altered stoichiometry of an evolved RNA aptamer

Inhibitory aptamers against a protein are promising as antagonistic reagents and repressive genetic components. Typically, improvement of such aptamers is achieved by acquiring higher binding affinity. Here, we report an alternative mechanism for the improvement of aptamer activity. Recently, we reported a transcriptional activator based on an inhibitory RNA aptamer against lambda cI repressor. We improved the aptamer through in vitro selection (SELEX) from a randomly mutagenized aptamer pool, followed by in vivo screening and truncation. Biochemical analyses indicated that the activity improvement was achieved by alteration of the complex formation stoichiometry, rather than by higher affinity or expression. Our results suggest an alternative strategy for improving aptamer activity.