MU2 and Double Strand Break Repair Figure 6. Interaction of MU2 with MRN complex. (A) Immunostaining of an Oregon R ovary with rabbit anti-RAD50 detected with secondary goat anti-rabbit IgG AlexaFluor 594 (red), and counterstained with DAPI (blue). (B) Germarium of an Oregon R ovary stained with rabbit anti-RAD50 (red), mouse anti-MU2 (green), and DAPI (blue) showing co-localization of RAD50 and MU2 in foci (yellow). (C) Immunoblot analysis of the eluates of the GST pull-down experiments detecting the physical interaction between the N-terminal FHA domain of MU2 and components of the MRN complex. (D) Genetic interaction between nbs and mu2 was assessed by studying the levels of apoptosis in Oregon R, homozygous nbs and mu2a mutants, and mu2a nbs double mutants. Wing imaginal discs were dissected, and propidium iodide staining was performed to detect apoptosis. doi:10.1371/journal.pgen.1000473.g006 chromosome breaks in the sperm pronuclei are generally repaired before syngamy, as evidenced by a paucity of mosaics among the mutants that are found [41]. If recognition of the damage by MU2 were necessary, then the repair of damage in the sperm should also be compromised in the absence of the MU2 protein, which however is not the case. What is the difference between radiation induced lesions in oocyte chromosomes and in sperm chromo- somes? There are at least two possibilities. One possibility is that sperm chromosomes are packaged using arginine-rich histones. Chromatin remodeling and eviction of nucleosomes is required for IRIF formation in standard chroma- tin. Such eviction of conventional nucleosomes may not be required in the case of sperm chromatin, as it is already devoid of the lysine-rich nucleosomes, and the arginine-rich histones must be stripped away to allow chromosome decondensation in prepara- tion for replication and syngamy. This may allow access of the induced lesion to repair enzymes in the absence of IRIFs. Alternatively, a DSB introduced into a sperm chromosome would be amenable to repair because the break produces two broken chromosome ends that would be maintained in the same nucleus and thus available for NHEJ or HR to restore the chromosome. A broken chromosome in a mature oocyte, on the other hand, would only put a single broken chromosome end into the zygote after the completion of the meiotic divisions, because PLoS Genetics | www.plosgenetics.org 9 May 2009 | Volume 5 | Issue 5 | e1000473