Crystal structures of monomeric BsmI restriction endonuclease reveal coordinated sequential cleavage of two DNA strands

BsmI, a thermophilic Type IIS restriction endonuclease from Bacillus stearothermophilus, presents a unique structural composition, housing two distinct active sites within a single monomer. Recognition of the non-symmetrical 5’-GAATGC-3’ sequence enables precise cleavage of the top and bottom DNA strands. Synthetic biology interventions have led to the transformation of BsmI into Nb.BsmI, a nicking endonuclease. Here we introduce Nt*.BsmI, tailored for top-strand cleavage, which is inactive on standard double-stranded DNA, but active on bottom-strand nicked DNA, suggesting a sequential cleavage mechanism. Crystallographic structures of pre- and post-reactive complexes with cognate DNA show one major conformational change, a retractable loop possibly governing sequential active site accessibility. The x-ray structures reveal the position of the divalent metal ions in the active sites and the DNA:protein interactions, while the models predicted by Alphafold3 are incorrect. This comprehensive structural and functional study lays a foundation for rational enzyme redesign and potential applications in biotechnology.

COMMUNICATIONS BIOLOGY

By: Rémi Sieskind, Sophia Missoury, Clément Madru, Isciane Commenge, Germain Niogret, Marcel Hollenstein, Yannick Rondelez, Ludovic Sauguet, Ahmed Haouz, Pierre Legrand and Marc Delarue.

Communications Biology volume 8, Article number: 387 (2025)

DOI: https://doi.org/10.1038/s42003-025-...