DNA double-strand breaks represent one of the most severe forms of DNA damage in mammalian cells. One pathway for repairing these breaks occurs via nonhomologous end-joining (NHEJ) and depends on XRCC4, LigaseIV, and Cernunnos, also called XLF. Although XLF stimulates XRCC4/LigaseIV to ligate mismatched and noncohesive DNA ends, the mechanistic basis for this function remains unclear. Here we report the structure of a partially functional 224 residue N-terminal fragment of human XLF. Despite only weak sequence similarity, XLF(1-170) shares structural homology with XRCC4(1-159). However, unlike the highly extended 130 A helical domain observed in XRCC4, XLF adopts a more compact, folded helical C-terminal region involving two turns and a twist, wrapping back to the structurally conserved N terminus. Mutational analysis of XLF and XRCC4 reveals a potential interaction interface, suggesting a mechanism for how XLF stimulates the ligation of mismatched ends.

DNA repair ͉ nonhomologous end-joining ͉ V(D)J recombinationT he nonhomologous end-joining (NHEJ) pathway is conserved in eukaryotes, from yeast to humans. Without requiring homologous DNA, NHEJ repairs DNA double-strand breaks produced by xenobiotic agents, such as topoisomerase II inhibitors and ionizing radiation, or by the cellular pathway for V(D)J recombination of the immunoglobulin genes (1). Even when the structure of the DNA ends prevents ligation, NHEJ processes the ends and repairs the breaks with high efficiency and minimal nucleotide loss. For ligatable ends, such as the blunt signal ends created by V(D)J recombination, NHEJ suppresses processing and repairs the breaks directly. Thus, NHEJ optimizes the preservation of DNA sequence, but the mechanism is not understood.

Biological processes that function chromosome-wide are not well understood. Here, we show that the Caenorhabditis elegans protein DPY-28 controls two such processes, X-chromosome dosage compensation in somatic cells and meiotic crossover number and distribution in germ cells. DPY-28 resembles a subunit of condensin, a conserved complex required for chromosome compaction and segregation. In the soma, DPY-28 associates with the dosage compensation complex on hermaphrodite X chromosomes to repress transcript levels. In the germline, DPY-28 restricts crossovers. In many organisms, one crossover decreases the likelihood of another crossover nearby, an enigmatic process called crossover interference. In C. elegans, interference is complete: Only one crossover occurs per homolog pair. dpy-28 mutations increase crossovers, disrupt crossover interference, and alter crossover distribution. Early recombination intermediates (RAD-51 foci) increase concomitantly, suggesting that DPY-28 acts to limit double-strand breaks (DSBs). Reinforcing this view, dpy-28 mutations partially restore DSBs in mutants lacking HIM-17, a chromatin-associated protein required for DSB formation. Our work further links dosage compensation to condensin and establishes a new role for condensin components in regulating crossover number and distribution. We propose that both processes utilize a related mechanism involving changes in higher-order chromosome structure to achieve chromosome-wide effects.[Keywords: X-chromosome dosage compensation; condensin; meiosis; crossover interference; epigenetics; Supplemental material is available at http://www.genesdev.org. Received September 24, 2007; revised version accepted November 15, 2007. No biological processes that operate on the level of an entire metazoan chromosome are understood in detail. We show that two such processes, X-chromosome dosage compensation and meiotic crossover (CO) control, are linked through a shared protein. Dosage compensation is an essential regulatory process that equalizes expression of most X-linked genes between the sexes (XO or XY males and XX females), despite their twofold difference in X-chromosome dose. CO number and distribution are tightly controlled during meiosis to ensure proper chromosome segregation. The use of a common protein suggests that a related mechanism underlies these two seemingly disparate chromosome-wide processes.In the nematode Caenorhabditis elegans, a dosage compensation complex (DCC) is targeted to both X chromosomes of hermaphrodites to reduce transcript levels by half, thereby achieving an X expression level in XX hermaphrodites similar to that in XO males (for review, see Meyer 2005). The DCC components DPY-27 and MIX-1 are members of the SMC (Structural Maintenance of Chromosomes) family of DNA-associated ATPases (Chuang et al. 1994;Lieb et al. 1998), and both resemble components of condensin, a highly conserved protein complex essential for the compaction, resolution, and segregation of mitotic and meiotic chromosomes from yeast to humans ...

Elevated blood ammonia (hyperammonemia) may cause delirium, brain damage, and even death. Effective treatments exist, but preventing permanent neurological sequelae requires rapid, accurate, and serial measurements of blood ammonia. Standard methods require volumes of 1 to 3 mL, centrifugation to isolate plasma, and a turn-around time of 2 h. Collection, handling, and processing requirements mean that community clinics, particularly those in low resource settings, cannot provide reliable measurements. We describe a method to measure ammonia from small-volume whole blood samples in 2 min. The method alkalizes blood to release gas-phase ammonia for detection by a fuel cell. When an inexpensive first-generation instrument designed for 100 μL of blood was tested on adults and children in a clinical study, the method showed a strong correlation (R 2 = 0.97) with an academic clinical laboratory for plasma ammonia concentrations up to 500 μM (16 times higher than the upper limit of normal). A second-generation hand-held instrument designed for 10–20 μL of blood showed a near-perfect correlation (R 2 = 0.99) with healthy donor blood samples containing known amounts of added ammonium chloride up to 1000 μM. Our method can enable rapid and inexpensive measurement of blood ammonia, transforming diagnosis and management of hyperammonemia.