The phenotypically similar hamster mutants irs1 and irs1SF exhibit high spontaneous chromosome instability and broad-spectrum mutagen sensitivity, including extreme sensitivity to DNA cross-linking agents. The human XRCC2 and XRCC3 genes, which functionally complement irs1 and irs1SF, respectively, were previously mapped in somatic cell hybrids. Characterization of these genes and sequence alignments reveal that XRCC2 and XRCC3 are members of an emerging family of Rad51-related proteins that likely participate in homologous recombination to maintain chromosome stability and repair DNA damage. XRCC3 is shown to interact directly with HsRad51, and like Rad55 and Rad57 in yeast, may cooperate with HsRad51 during recombinational repair. Analysis of the XRCC2 mutation in irs1 implies that XRCC2's function is not essential for viability in cultured hamster cells.

The Ångström-scale transport characteristics of water and six different solutes, methanol, ethanol, 2propanol, urea, Na + , and Cl -, were studied for a polymeric reverse osmosis (RO) membrane, FT-30, using non-equilibrium molecular dynamics (NEMD) simulations. Results indicate that water transport increases with an increasing fraction of percolated free volume, or water-accessible open space, in the membrane polymer structure. The trajectories of solute molecules display Brownian motion and hop from pore to pore as they pass through the polymer chain structure of the membrane. The solute transport depends on both the Van der Waals size of the dehydrated solute and the electrostatic interaction of the solute with water and the membrane. For alcohol solutes, transport decreases for solutes with larger Van der Waals volume, which corresponds to less available percolated free volume, or solute-accessible space, within the membrane polymer structure. Urea has reduced transport compared ethanol, most likely due to more complex chemistry or polarity than the alcohol solutes, even though urea has a smaller Van der Waals volume than ethanol. Na + and Clexperience the lowest transport, likely due to strong ion-water and ionion electrostatic interactions. NEMD simulations provide a unique opportunity to understand molecular level mechanisms for water and solute transport in polymeric RO membranes for water purification.

The molecular basis for the DNA repair dysfunction observed in mutant Chinese hamster ovary cell lines of X-ray repair cross complementing group 1 (XRCC1) is unknown and the exact role of the XRCC1 protein remains unclear. To help clarify the role of the XRCC1 gene we analyzed four mutant cell lines of this complementation group and a revertant cell line for XRCC1 protein content and for sequence alterations in the XRCC1 coding region. Immunoblot analysis of cellular extracts indicated that each of four mutant lines was lacking XRCC1 protein, whereas the repair-proficient revertant line derived from one of these mutants contained a normal level of XRCC1. Although each of these cell lines expressed XRCC1 mRNA, we found in all cases a distinct point mutation resulting in crucial alterations in the encoded XRCC1 protein sequence of 633 amino acids. Two of the mutations cause non-conservative amino acid changes, Glu102-->Lys and Cys390-->Tyr, at positions that are invariant among hamster, mouse and human XRCC1 sequences and are located in putative functional domains. A third debilitating mutation disrupts RNA splicing, generating multiple transcripts of different length that contain deletions spanning a region of >100 amino acids in the midsection of the XRCC1 coding sequence. A fourth mutation results in a termination codon that shortens the open reading frame to 220 amino acids, however, in the revertant cell line a further mutation in the same codon, Stop221-->Leu, permits translation of a full-length functional variant protein. These mutational data indicate the importance of the putative functional regions in XRCC1, such as the BRCA1 C-terminal (BRCT) domain found in common with BRCA1 and other DNA repair and cell cycle checkpoint proteins, and also regions necessary for interaction with DNA polymerase beta and DNA ligase III.

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We use molecular dynamics simulations to compute junction thermal conductance of carbon nanotubes as a function of crossing angle and pressure, and conductivity of arrays and bundles consisting of multiple junctions as a function of pressure. Two types of arrays are investigated: crossbar structures consisting of alternating orthogonal layers of nanotubes and close-packed bundles of parallel oriented tubes. Conductance of 90° junction increases with pressure 4 fold before saturation; cross-plane thermal conductivity of crossbar structures increases by a factor of 2. For parallel junctions pressure doubles the conductance while thermal conductivity of nanotubes bundles is more or less pressure independent.