Structural insights into lab-coevolved RNA–RBP pairs and applications of synthetic riboswitches in cell-free system

CS1-LS4 and CS2-LS12 are ultra-high affinity and orthogonal RNA-protein pairs that were identified by PD-SELEX (Phage Display coupled with Systematic Evolution of Ligands by EXponential enrichment). To investigate the molecular basis of the lab-coevolved RNA-RBP pairs, we determined the structures of the CS1-LS4 and CS2-LS12 complexes and the LS12 homodimer in an RNA-free state by X-ray crystallography. The structural analyses revealed that the lab-coevolved RNA-RBPs have acquired unique molecular recognition mechanisms, whereas the overall structures of the RNP complexes were similar to the typical kink-turn RNA-L7Ae complex. The orthogonal RNA-RBP pairs were applied to construct high-performance cell-free riboswitches that regulate translation in response to LS4 or LS12. In addition, by using the orthogonal protein-responsive switches, we generated an AND logic gate that outputs staphylococcal γ-hemolysin in cell-free system and carried out hemolysis assay and calcein leakage assay using rabbit red blood cells and artificial cells, respectively.