Chimeric archaeal BHB-human pre-tRNA scaffold HARBOR system enables versatile production of functional circular RNA modalities

Circular RNAs (circRNAs) exhibit superior intrinsic stability and lower immunogenicity compared with linear RNAs, rendering them promising modalities for biomedical and biotechnological applications. Here, we establish the HARBOR (Human-Archaeal RNA Backbone for Overexpression of circular RNA) system, an engineered endogenous circRNA expression platform integrating a human precursor transfer RNA (pre-tRNA) scaffold with an archaeal Bulge-Helix-Bulge (BHB) motif. HARBOR repurposes the endogenous tRNA splicing machinery to generate chimeric pre-tRNA intermediates, which are rapidly degraded following processing, while producing circRNAs ranging from hundreds of nucleotides to 8 knt in cells and in vivo. Notably, it enables robust production of large-sized circRNAs with yields nearly 80-fold than the intronic complementary sequence-mediated back-splicing (3D5) system and can drive protein expression from translatable circRNAs at an efficiency close to 20-fold than the Tornado system. Such prominent properties support diverse applications of HARBOR, including producing therapeutic aptamers targeting PKR-associated neuroinflammation and pathological TDP-43 condensation, as well as protein-coding circRNAs encoding adenine base editors for genome editing and MYOD1 for fibroblast-to-myocyte trans-differentiation. Collectively, HARBOR represents a versatile, scalable, and user-friendly platform that unifies circRNA-based aptamer and protein expression applications, advancing circRNA-enabled engineering and biomedical applications.