Figure 2.

Molecular mechanism of Polymerase theta mediated end joining. a) Pol θ has an N-terminal superfamily 2 helicase-like domain (HelD, cyan) separated from a C-terminal A family Polymerase domain (PolD, yellow) by a poorly conserved central domain (CenD, black); domain boundaries are for the human Pol θ protein (Uniprot O75417). b) Model for steps in TMEJ, identifying at right the domains of Pol θ or other factors argued to perform these steps. Hydrolysis of ATP (ATPase) by the HelD removes Replication protein A (RPA) from resected ends to enable subsequent steps, and may drive sampling of end pairings to find complementary sequence (in red) at microhomologies before a nuclease (scissors) removes non-homology tails to activate synthesis (in blue) by the PolD. Resolution could occur without strand displacement (short patch) or with strand displacement (long patch) c) Mechanism for generation of templated insertions (TINS) through aborted intermolecular synthesis to generate direct repeat TINS, or aborted intramolecular synthesis primed by annealing of inverted repeats to generate inverted repeat TINS. Complementary sequences at microhomologies are in red, inserted sequence in junctions is bolded, and the orientation of inserted sequence relative to template is noted by blue arrows. Intramolecular synthesis cannot directly contribute to productive repair, and may be blocked by the nuclease activity of the PolD (scissors) and by unwinding activity of the HelD.