Recombination was measured in Chinese hamster ovary (CHO-Kl) cells and in the X-ray-sensitive mutants xrsl and xrs7, which show a defect in DNA double-strand break repair. To assay recombination, pairs of derivatives of the plasmid pSV2gpt were constructed with nonoverlapping deletions in the gpt gene region and cotransferred into the dilTerent cell types. Recombination efficiencies, measured as the transformation frequency with a pair of deletion plasmids relative to that with the complete pSV2gpt plasmid, were about 6% in both CHO-Kl and the xrs mutants for plasmids linearized at a site outside the gpt gene. However, these efficiencies were substantially enhanced by the introduction of a double-strand break into the homologous region of the gpt gene in one of a pair of deletion plasmids before cotransfer. This enhancement was apparently only about half as great for the xrs cells as for CHO-Kl, but variation In microbes, a genetic link has been established between ionizing-radiation resistance and recombination proficiency; prominent examples of the genes involved are found in the RecF pathway of Escherichia coli (41) and in the RAD52 group of Saccharomyces cerevisiae (14). Additionally, mutation of such genes often affects the ability to repair DNA double-strand breaks. Thus, it has been hypothesised that double-strand breaks induced by ionizing radiation are rejoined by a process of recombination between homologous chromosomal regions (24).To investigate the relationship between ionizing-radiation resistance and recombination in mammalian cells, we have examined the recombination proficiencies of wild-type and X-ray-sensitive (xrs) hamster cells. The xrs mutants were isolated from a Chinese hamster ovary (CHO) cell line by Jeggo and Kemp (10). These mutants lack the ability to recover from potentially lethal radiation damage (38) and show reduced repair of DNA double-strand breaks (11, 43). The two mutants selected for this study, xrsl and xrs7, belong to the same complementation group (9) and differ only slightly in their responses to ionizing radiations (10, 38), but show substantially different responses to other genotoxic agents (10).The study of recombination in mammalian cells has been facilitated in the last few years by the use of virus-and plasmid-based gene transfer methods. It has been shown that mammalian cells have the enzymatic processes necessary for both homologous and nonhomologous recombination of introduced DNA (6,12,23,27,31,32,34,39,40,44). We have attempted to apply these methods quantitatively by carefully controlling DNA transfer conditions to determine plasmid recombination efficiencies in the parent and mutant CHO cells. Additionally we have followed up the observation, made initially for plasmid-chromosome recombination in S. cerevisiae (21), that a double-strand break in regions of * Corresponding author. homology greatly stimulates recombination. While our experiments were in progress on plasmid-plasmid recombination in CHO cells, it was reported that double-strand breaks are...

a b s t r a c tThe urease enzyme (Urs) was successfully incorporated into a polypyrrole film (PPy) in one simple electropolymerisation step. The films were formed using different dopant anions. The polypyrrole-urease-chloride film (PPy-Urs-Cl) was deposited from a simple chloride dopant, and the polypyrrole-urease-sulfonated-␤-cyclodextrin (PPy-Urs-SCD) was formed using a sulfonated-␤-cyclodextrin (SCD) dopant. The presence of the Urs within the polymer film was evident from the fibrous morphology, observed in the SEM micrographs, and the presence of nickel, arising from the active site of the urease enzyme. The sensing ability of the films and their enzyme-free counterparts (i.e., PPy-Cl and PPy-SCD) towards urea was investigated. The dopant anion plays an important role in the sensitivity of the polymer films towards urea. The PPy-Urs-SCD film has a superior sensitivity of 5.79 C M −1 compared to 0.76 C M −1 for the PPy-Urs-Cl polymer film. Furthermore, the negative groups on the SCD eliminate interference from common interfering compounds, such as ascorbic acid.

Interferon-alpha (IFN-alpha), in conjunction with ribavirin, is the current standard for the treatment of chronic hepatitis C virus (HCV) infection. This treatment requires frequent dosing, with a significant risk of the development of anti-IFN-alpha neutralizing antibodies that correlates with lack of efficacy or relapse. We have developed an IFN-alpha linked to the Fc region of human IgG1 for improved half-life and less frequent dosing. We have also identified, using a human T cell proliferation assay, three regions of IFN-alpha2b that are potentially immunogenic, and a variant containing a total of six mutations within these regions was made. This variant was made as a fusion to Fc either with or without a flexible linker between the fusion partners. Both configurations of the variant were less active than native IFN-alpha alone, although the variant containing the flexible linker had in vitro antiviral activity within the range of other modified IFN-alphas currently in clinical use. Peptides spanning the modified regions were tested in T cell proliferation assays and found to be less immunogenic than native controls when using peripheral blood mononuclear cells (PBMCs) from both healthy individuals and HCV-infected patients who had been treated previously with IFN-alpha2b.

a b s t r a c tA highly sensitive urea sensor was developed by incorporating the urease enzyme (Urs) into a polypyrrole film (PPy) in one simple electropolymerisation step, using a sulfonated-␤-cyclodextrin dopant. This PPyUrs-SCD film has a superior sensitivity of 5.79 C M −1 and detection in the region of 1.0 × 10 −10 M urea, which is greater than other urea sensors reported in the literature. This is due to the formation of an inclusion complex between urea and a sulfonated-␤-cyclodextrin host in an aqueous solution, which was established using electrochemical techniques. Cyclic voltammetry was used to investigate the effect of an excess concentration of the sulfonated-␤-cyclodextrin on the currents recorded for urea. A clear reduction in the current was observed upon the addition of the sulfonated-␤-cyclodextrin. The formation constant, K f , was computed as 2745 ± 300 M −1 , indicating the formation of a relatively strong inclusion complex. In addition, a 1:1 stoichiometry for the inclusion complex was deduced from a Job's plot analysis.

A humanized monoclonal antibody that binds to the 86-kDa glycoprotein, gpUL75 (gH), of human cytomegalovirus (CMV) has been developed. The six complementarity determining regions of the heavy and light chains of the mouse antibody HCMV16 were transferred to human antibody framework sequences and combined with human antibody constant regions to produce a complete antibody. The reshaped antibody recognized cells infected with a variety of virus strains and neutralized clinical isolates of CMV as efficiently as laboratory strains in a conventional plaque reduction assay. This antibody provides a potential agent for the prevention or treatment of CMV infections in humans.