The MRX Complex Ensures NHEJ Fidelity through Multiple Pathways Including Xrs2-FHA–Dependent Tel1 Activation

Because DNA double-strand breaks (DSBs) are one of the most cytotoxic DNA lesions and often cause genomic instability, precise repair of DSBs is vital for the maintenance of genomic stability. Xrs2/Nbs1 is a multi-functional regulatory subunit of the Mre11-Rad50-Xrs2/Nbs1 (MRX/N) complex, and its function is critical for the primary step of DSB repair, whether by homologous recombination (HR) or non-homologous end joining. In human NBS1, mutations result truncation of the N-terminus region, which contains a forkhead-associated (FHA) domain, cause Nijmegen breakage syndrome. Here we show that the Xrs2 FHA domain of budding yeast is required both to suppress the imprecise repair of DSBs and to promote the robust activation of Tel1 in the DNA damage response pathway. The role of the Xrs2 FHA domain in Tel1 activation was independent of the Tel1-binding activity of the Xrs2 C terminus, which mediates Tel1 recruitment to DSB ends. Both the Xrs2 FHA domain and Tel1 were required for the timely removal of the Ku complex from DSB ends, which correlates with a reduced frequency of imprecise end-joining. Thus, the Xrs2 FHA domain and Tel1 kinase work in a coordinated manner to maintain DSB repair fidelity.     

Sexing schistosome larvae

Biological and immunological factors may influence changes in sex ratios at different points of the schistosome life cycle, resulting in the fact that female schistosomes are significantly outnumbered by males in chronic infections of snails and mammalian hosts. Analysis of this phenomenon has long been hampered by shortcomings in the methods used to determine sex ratios. Here, Robin Gasser describes recently developed molecular methods for sexing schistosome larvae. These have opened the way towards understanding why sex ratios become male biased and allow a proper assessment of its consequences in the epidemiology, diagnosis and pathology of infection.     

Molecular and phylogenetic characterisation of Cryptosporidium from birds

Avian isolates of Cryptosporidium species from different geographic locations were sequenced at two loci, the 18S rRNA gene and the heat shock gene (HSP-70). Phylogenetic analysis of the sequence data provided support for the existence of a new avian species of Cryptosporidium infecting finches and a second species infecting a black duck. The identity of Cryptosporidium baileyi and Cryptosporidium meleagridis as valid species was confirmed. Also, C. baileyi was identified in a number of isolates from the brown quail extending the host range of this species.     

Modulation of drug sensitivity in yeast cells by the ATP-binding domain of human DNA topoisomerase IIalpha

Epipodophyllotoxins are effective antitumour drugs that trap eukaryotic DNA topoisomerase II in a covalent complex with DNA. Based on DNA cleavage assays, the mode of interaction of these drugs was proposed to involve amino acid residues of the catalytic site. An in vitro binding study, however, revealed two potential binding sites for etoposide within human DNA topoisomerase IIα (htopoIIα), one in the catalytic core of the enzyme and one in the ATP-binding N-terminal domain. Here we have tested how N-terminal mutations that reduce the affinity of the site for etoposide or ATP affect the sensitivity of yeast cells to etoposide. Surprisingly, when introduced into full-length enzymes, mutations that lower the drug binding capacity of the N-terminal domain in vitro render yeast more sensitive to epipodophyllotoxins. Consistently, when the htopoIIα N-terminal domain alone is overexpressed in the presence of yeast topoII, cells become more resistant to etoposide. Point mutations that weaken etoposide binding eliminate this resistance phenotype. We argue that the N-terminal ATP-binding pocket competes with the active site of the holoenzyme for binding etoposide both in cis and in trans with different outcomes, suggesting that each topoisomerase II monomer has two non-equivalent drug-binding sites.     

Cutting Edge: Multiple autoimmune pathways in kd/kd mice

The kidney disease (kd) mutation was transferred to a C57BL/6 (B6) background by selection for closely linked microsatellite markers. The resulting congenic strain, B6.kd, was mated with partners homozygous for targeted mutations of CD4, CD8, CD28, IL-2, recombinase-activating gene-1 (Rag-1), ICAM-1, or beta(2)-microglobulin. In most of the resulting double mutants, kidney disease occurred as readily and as severely as in the B6.kd controls, although disease occurred somewhat less frequently in age-matched CD28(-/-) kd/kd mice. Immunohistology demonstrated a predominance of macrophages in the lesions of B6.kd and most of the double mutants, with the remaining cells consisting of T cells and variable numbers of NK cells. In Rag-1(-/-) kd/kd, approximately 50% of infiltrating cells were macrophages, and approximately 50% were NK cells. These results suggest that the initial lesion caused by the mutant gene is intrinsic to the kidney and that the immune response that subsequently occurs can involve any one of several different cellular compositions.