FIGURE 5:

Schematic diagram of possible Zhp3 involvement in the CO–NCO decision in Tetrahymena. The processes shown are based on models proposed for the protection of meiotic joint molecules (JMs) by Msh-Msh5 (MutSγ) complexes (Snowden et al., 2004) and their stabilization by Zip3/RNF2012 family members (Reynolds et al., 2013). Extension of the heteroduplex by Mer3 (Mazina et al., 2004) is not shown because there is no evidence for this activity in Tetrahymena. Of the two single-stranded 3′ ends generated by strand resection around a DSB (A), one invades a DNA molecule for homology searching. Once a homologous region is found and a short heteroduplex has been formed, the Msh5-Msh5 (MutSγ) complex may associate with a JM (B). Once the invading end starts to extend by repair synthesis (which can be observed cytologically by BrdU incorporation), MutSγ is stabilized by Zhp3, allowing it to efficiently protect the JM (C). Zhp3 foci only rarely localize before BrdU incorporation. Protected JMs will then capture the second 3′ end and form COs (D). Most unprotected JMs are unwound and become NCOs via the SDSA pathway (E, F). However, because the absence of Zhp3 or MutSγ does not completely abolish CO formation, some unprotected JMs might persist for long enough to allow second-end capture to occur (G).