A closed conformation for the Pol λ catalytic cycle

Pol λ is a family X member believed to fill short gaps during DNA repair. Here we report crystal structures of Pol λ representing three steps in filling a single-nucleotide gap. These structures indicate that, unlike other DNA polymerases, Pol λ does not undergo large subdomain movements during catalysis, and they provide a clear characterization of the geometry and stereochemistry of the in-line nucleotidyl transfer reaction.