Structure-function analysis and exonuclease deletion yield an improved strand-displacing DNA polymerase from...

Isothermal nucleic-acid amplification relies on strand-displacing DNA polymerases that synthesize DNA while unwinding duplex templates. We previously identified two thermostable family-A polymerases, C1-Pol from Geobacillus zalihae and H1-Pol from Aeribacillus pallidus, which show a characteristic and reproducible trade-off between stability and activity. C1-Pol is more thermostable, whereas H1-Pol exhibits stronger strand-displacement activity and higher recombinase polymerase amplification (RPA) efficiency. Domain-swapping analysis indicated that a chimeric construct (C1-Pol#4), in which the C1-Pol 5'→3' exonuclease region was replaced with that of H1-Pol, enhanced strand-displacement and nucleotide-incorporation activities while maintaining parental-level stability, suggesting that this region modulates these important biochemical properties. However, C1-Pol#4 did not improve RPA efficiency. To clarify the functional contribution of this domain, we constructed Klenow-type deletion mutants lacking the 5'→3' exonuclease region. The H1-Pol large fragment (H1-PolLF) exhibited reduced thermostability but markedly enhanced polymerase and strand-displacement activities at 40 °C, the optimal temperature for RPA reactions. At this temperature, H1-PolLF showed ∼10-fold higher strand-displacement activity than Bst DNA polymerase 2.0 and 4.4-fold higher than full-length H1-Pol. H1-PolLF also accelerated RPA, producing equivalent amplicons in half the reaction time and improving detection sensitivity 100-fold (6 × 10³ → 6 × 10¹ copies) than parental H1-Pol. Coupling H1-PolLF with branched-chain polyamine (BCPA)-conjugated magnetic beads enabled reliable detection of ∼10² target molecules from 10 mL saline. These findings demonstrate that removing the 5'→3' exonuclease domain fine-tunes polymerase and strand-displacing functions, yielding an enzyme highly suited for rapid, ultrasensitive isothermal nucleic-acid detection when used in combination with BCPA beads.