Riboswitches, noncoding RNAs that bind a small molecule effector to control gene expression at the level of transcription or translation, are uniquely suited to meet challenges in synthetic biology. To expand the limited set of existing riboswitches, we developed a riboswitch discovery platform that couples dual genetic selection and fluorescence-activated cell sorting to identify novel riboswitches from a 108 random-sequence library in which the aptamer domain of the ThiM#2 riboswitch was replaced with an N40 sequence. In a proof-of-principle validation, we identified novel riboswitches for the small molecule theophylline. Our best riboswitch (Hit 3–5) displays 2.3-fold activation of downstream gene expression in the presence of theophylline. Random mutagenesis of Hit 3–5, coupled with selections and screens, afforded improved riboswitches displaying nearly 3-fold activation. To the best of our knowledge, this is the first report of in vivo directed evolution of an aptamer domain to generate a functional riboswitch.

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Highlights d Unassembled subunits of membrane protein complexes must be recognized and degraded d The obligate hetero-oligomer WRB/CAML has differential modes of orphan recognition d WRB is inserted correctly independently of CAML but is degraded when unassembled d CAML requires WRB to fold correctly, which prevents exposure of a degron

The ability to map genetic interactions has been essential for determining gene function and defining biological pathways. Therefore, a system to readily perform genome-wide genetic modifier screens in human cells is a powerful platform for dissecting complex processes in mammalian cells, where redundancy and adaptation commonly mask the phenotype of a single genetic perturbation. Here, we report a CRISPR interference (CRISPRi) based platform, compatible with Fluorescence Activated Cell Sorting (FACS)-based reporter screens, that can be used to query epistatic relationships at scale. This is enabled by a flexible dual-sgRNA library design that allows for the simultaneous delivery and selection of a fixed sgRNA and a second randomized guide, comprised of a genome-wide library, with a single transduction. As a proof of principle, we apply our approach to study the pathways that mediate tail-anchored (TA) protein insertion at the endoplasmic reticulum (ER). We show that this dual-guide library approach can be successfully coupled with FACS-based reporter screening, to identify genetic epistasis and thereby place TA biogenesis factors in their respective parallel pathways. We demonstrate that this dual-guide approach is both more sensitive and specific than traditional growth screening approaches, and is ideally suited for dissecting the complex interplay between factors in human cells.

In vitro diagnostics (IVD) have become increasingly popular tool for tackling global health concerns, but conventional IVD tests, such as microscopy and nucleic‐acid amplification methods, are often expensive and complex to use, limiting their use in low‐resource settings. Whole‐cell biosensors in bacterial systems offer an attractive solution as an easy‐to‐use, portable device that non‐specialists could use for clinical purposes outside clinical locations. However, a strategy is needed to engineer ligand specific sensors for new targets. An appealing candidate for a sensing module is the riboswitch. Genetic RNA switches, termed riboswitches, are genetic regulatory elements found in the 5′‐UTR (untranslated region) of some prokaryotic mRNA and contain two functional components: an aptamer binding domain and an expression platform that modulates downstream gene expression. We developed an engineering platform coupling genetic selections and Fluorescence‐Activated Cell Sorting (FACS) screens to identify novel riboswitches from a 108 random‐sequence library in which we replaced the aptamer sequence of the ThiM#2 riboswitch with a degenerate 40‐nucleotide sequence. In a proof‐of‐principle validation of the platform, we identified novel riboswitches for the small molecule theophylline, which are distinct from the existing synthetic theophylline switches. Our best riboswitch hit displays a 2.3‐fold activation in response to theophylline and does not respond to the structurally unrelated molecule thiamine. We performed directed evolution on this riboswitch hit by mutagenizing its sequence and performing additional rounds of selections and screens, resulting in a variant with a 3.6‐fold activation. Current efforts are focused on assessing the efficacy of the selection platform when isolating riboswitches for dopamine. Furthermore, a variant of the engineering platform is being used side‐by‐side to compare if a dopamine riboswitch can be selected from a library composed of an in vitro selected aptamer coupled with a 15‐nucleotide degenerate sequence. We anticipate that these efforts will provide general guidelines toward the development of novel riboswitches for a variety of natural and non‐natural ligands.Support or Funding InformationNSF CBET 1258307Arnold and Mabel Beckman FoundationThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.