ESPeR-seq: Extremely Sensitive and Pure, End-to-end, RNA-seq library preparation.

The Smart-seq family of methods represents the gold standard for high-sensitivity, full-length single-cell RNA sequencing. Despite iterative improvements, fundamental challenges remain: the generation of non-specific PCR products that limit sensitivity, the inability to capture precise Transcription End Sites (TES), and the insidious generation of "phantom UMIs"-artificial molecular barcodes created during PCR that systematically inflate molecular counts. Here, we present ESPeR-seq, a novel architecture that resolves these barriers. To enable precise, stranded TES capture, we developed an "Omega-dT" primer that bypasses synthetic poly-T tracts, restoring high-quality sequencing directly at transcript termini. To eliminate both PCR background and phantom UMIs, we implemented a biochemical "multi-lock" mechanism utilizing uracil-containing TSOs and a uracil-intolerant DNA polymerase. We validate this approach using the logQ-slope, a novel metric that sensitively diagnoses UMI fidelity. Benchmarking reveals that while state-of-the-art methods still exhibit signs of UMI inflation, ESPeR-seq strictly prevents it. Furthermore, the strandedness and precise end-delineation provided by TSO and dT reads support robust