MU2 and Double Strand Break Repair Figure 7. Structure of MU2. Diagram of the MU2 protein showing the N-terminal FHA domain and the C-terminal tandem BRCT domain. Arrows show the positions of threonine-glutamine (TQ) dipeptides. Numbers indicate the amino acid positions. Homology to MDC1 is based on a blast2 comparison using BLOSUM45. doi:10.1371/journal.pgen.1000473.g007 the acentric fragment would not move to the pole at anaphase and would be lost. Thus, this single broken chromosome end in the oocyte pronucleus would not be amenable for either NHEJ or HR. As there is no cell cycle control in the early embryo [42], the broken chromosome ends may have a chance to associate with the telomeric proteins HP1 and HOAP, which are present in abundance in the embryo, and become established as new telomeres (Figure 9). After syngamy the new diploid nucleus undergoes a rapid series of divisions to produce a thousand nuclei with 16,000 telomeres within 1–2 hours. Thus, the young embryo seems poised to generate a large number of new telomeric ends in a very short time. This may explain why an unrepaired chromosome break may acquire a new telomere at a high rated during development of the young embryo. This result is reminiscent of work in maize, where broken chromosome ends are healed specifically upon entry into the zygote [10]. Our observations provide a biochemical and a molecular basis for this phenotype. Figure 8. MU2 is involved in the intra S-phase checkpoint. (A) Eye imaginal discs from Oregon R and mu2a larvae were treated with 0.05 M HU for 2.5 h. Mitotic cells were assessed by staining the discs with the mitosis-specific marker phospho-histone 3 (PH3) Ser 10 followed by fluorescence microscopy. (B) Plot showing the number of mitotic cells in each genotype in the presence and absence of HU. Five discs were assayed for the number of mitotic cells. The mu2a mutant was assayed in two different genetic backgrounds. doi:10.1371/journal.pgen.1000473.g008 PLoS Genetics | www.plosgenetics.org 10 May 2009 | Volume 5 | Issue 5 | e1000473