Figure 9.

Model for the role of MRN-Ctp1 in NHEJ repair of Hermes excision sites. The initial hairpin-capped ends are bound by Ku, which has end protection functions that may prevent further processing (Ramsden and Gellert 1998; Smider et al. 1998; Arosio et al. 2002). MRN-Ctp1 is proposed to bind and replace Ku and allow synapsis to bridge the two DNA ends [as in Langerak et al. (2011) and Limbo et al. (2011)] and to recruit factors that open the hairpin. Subsequent base pairing at microhomologies (yellow box) may be potentiated by MRN or other factors prior to ligation. While MRN has a known role in strand resection (McVey and Lee 2008; Nicolette et al. 2010; Paull 2010), the most frequently observed end-joining events (Figure 3) indicated that little or no strand resection occurred to expose homologies. MRN, Mre11-Rad50-Nbs1; NHEJ, nonhomologous end-joining; wild-type, WT.