Figure 7.

A model for the roles of Exo1 in accurate joining of cohesive ends. DNA ends with 5′-extensions are potentially substrates for filling in by Pol4 or another DNA polymerase. We previously suggested that the filling reaction might be largely prevented in the presence of active Tdp1, due to the removal of a nucleoside, and the generation of a 3′-phosphate (this reaction is not shown for simplicity). If Tdp1 does not act, and a DNA polymerase fills in the extension, synapsis and isomerization can lead to a structure with a 5′-flap. The flap can be removed by either the 5′→3′-exonuclease or the flap endonuclease activity of Exo1, leading to the recovery of an error-free product. The filling-in reaction cannot be directly reversed by Exo1, since it does not have 3′→5′-exonuclease activity.