Kuehnl, F. | bioRxiv Science

bioRxiv2020

Beyond Plug and Pray: Context Sensitivity and in silico Design of Artificial Neomycin Riboswitches

Gene regulation in prokaryotes often depends on RNA elements such as riboswitches or RNA thermometers located in the 5’ untranslated region of mRNA. Rearrangements of the RNA structure in response, e. g., to the binding of small molecules or ions control translational initiation or premature termination of transcription and thus mRNA expression. Such structural responses are amenable to computational modeling, making it possible to rationally design synthetic riboswitches for a given aptamer. Starting from an artificial aptamer, we construct the first synthetic transcriptional riboswitches that respond to the antibiotic neomycin. We show that the switching behavior in vivo critically depends not only on the sequence of the riboswitch itself, but also on its sequence context. We therefore developed in silico methods to predict the impact of the context, making it possible to adapt the design and to rescue non-functional riboswitches. We furthermore analyze the influence of 5’ hairpins with varying stability on neomycin riboswitch activity. Our data highlight the limitations of a simple plug-and-play approach in the design of complex genetic circuits and demonstrate that detailed computational models significantly simplify, improve, and automate the design of transcriptional circuits. Our design software is available under a free license on Github.1Competing Interest StatementThe authors have declared no competing interest.List of Abbreviationsauarbitrary time unitsAURaptamer upstream regionbgaBβ-galactosidasebgaBthe β-galactosidase genedLdecoupling leaderE. coliEscherichia colieGFPthe enhanced green fluorescent proteinegfpthe enhanced green fluorescent protein geneGFPgreen fluorescent proteinLHleader hairpinLMmutated leaderMFEminimum free energymRNAmessenger RNAoLoriginal leaderPospositive controlRBSribosomal binding siteRFUrelative fluorescence unitRNAribonucleic acidRNAPRNA polymeraseRNase ERibonuclease ERppHRNA 5’ pyrophosphohydrolaserRNAribosomal RNASELEXsystematic evolution of ligands by exponential enrichmentSSCsaline sodium citrate bufferTBETris-borate-EDTATDRterminator downstream regionTSStranscription start siteUunstructured regionUTRuntranslated regionView Full Text

synthetic biology↗

bioRxiv2020

Assessing the Quality of Cotranscriptional Folding Simulations

Structural changes in RNAs are an important contributor to controlling gene expression not only at the post-transcriptional stage but also during transcription. A subclass of riboswitches and RNA thermometers located in the 5 region of the primary transcript regulates the downstream functional unit - usually an ORF - through premature termination of transcription. Such elements not only occur naturally but they are also attractive devices in synthetic biology. The possibility to design such riboswitches or RNA thermometers is thus of considerable practical interest. Since these functional RNA elements act already during transcription, it is important to model and understand the dynamics of folding and, in particular, the formation of intermediate structures concurrently with transcription. Cotranscriptional folding simulations are therefore an important step to verify the functionality of design constructs before conducting expensive and labour-intensive wet lab experiments. For RNAs, full-fledged molecular dynamics simulations are far beyond practical reach both because of the size of the molecules and the time scales of interest. Even at the simplified level of secondary structures further approximations are necessary. The BarMap approach is based on representing the secondary structure landscape for each individual transcription step by a coarse-grained representation that only retains a small set of low-energy local minima and the energy barriers between them. The folding dynamics between two transcriptional elongation steps is modeled as a Markov process on this representation. Maps between pairs of consecutive coarse-grained landscapes make it possible to follow the folding process as it changes in response to transcription elongation. In its original implementation, the BarMap software provides a general framework to investigate RNA folding dynamics on temporally changing landscapes. It is, however, difficult to use in particular for specific scenarios such as cotranscriptional folding. To overcome this limitation, we developed the user-friendly BarMap-QA pipeline described in detail in this contribution. It is illustrated here by an elaborate example that emphasizes the careful monitoring of several quality measures. Using an iterative workflow, a reliable and complete kinetics simulation of a synthetic, transcription regulating riboswitch is obtained using minimal computational resources. All programs and scripts used in this contribution are free software and available for download as a source distribution for Linux(R), or as a platform-independent Docker(R) image including support for Apple macOS(R) and Microsoft Windows(R).

bioinformatics↗