XRCC3 promotes homology-directed repair of DNA damage in mammalian cells

Homology-directed repair of DNA damage has recently emerged as a major mechanism for the maintenance of genomic integrity in mammalian cells. The highly conserved strand transferase, Rad51, is expected to be critical for this process. XRCC3 possesses a limited sequence similarity to Rad51 and interacts with it. Using a novel fluorescence-based assay, we demonstrate here that error-free homology-directed repair of DNA double-strand breaks is decreased 25-fold in an XRCC3-deficient hamster cell line and can be restored to wild-type levels through XRCC3 expression. These results establish that XRCC3-mediated homologous recombination can reverse DNA damage that would otherwise be mutagenic or lethal.

The reservoir for HIV-1 in human peripheral blood is a T cell that maintains expression of CD4

Human immunodeficiency virus type 1 (HIV-1) selectively infects cells expressing the CD4 molecule, resulting in substantial quantitative and qualitative defects in CD4+ T lymphocyte function in patients with acquired immunodeficiency syndrome (AIDS). However, only a very small number of cells in the peripheral blood of HIV-1-infected individuals are expressing virus at any given time. Previous studies have demonstrated that in vitro infection of CD4+ T cells with HIV-1 results in downregulation of CD4 expression such that CD4 protein is no longer detectable on the surface of the infected cells. In the present study, highly purified subpopulations of peripheral blood mononuclear cells (PBMCs) from AIDS patients were obtained and purified by fluorescence-automated cell sorting. They were examined with the methodologies of virus isolation by limiting dilution analysis, in situ hybridization, immunofluorescence, and gene amplification. Within PBMCs, HIV-1 was expressed in vivo predominantly in the T cell subpopulation which, in contrast to the in vitro observations, continued to express CD4. The precursor frequency of these HIV-1-expressing cells was about 1/1000 CD4+ T cells. The CD4+ T cell population contained HIV-1 DNA in all HIV-1-infected individuals studied and the frequency in AIDS patients was at least 1/100 cells. This high level of infection may be the primary cause for the progressive decline in number and function of CD4+ T cells in patients with AIDS.

Chromosome instability and defective recombinational repair in knockout mutants of the five Rad51 paralogs

The Rad51 protein, a eukaryotic homologue of Escherichia coli RecA, plays a central role in both mitotic and meiotic homologous DNA recombination (HR) in Saccharomyces cerevisiae and is essential for the proliferation of vertebrate cells. Five vertebrate genes, RAD51B, -C, and -D and XRCC2 and -3, are implicated in HR on the basis of their sequence similarity to Rad51 (Rad51 paralogs). We generated mutants deficient in each of these proteins in the chicken B-lymphocyte DT40 cell line and report here the comparison of four new mutants and their complemented derivatives with our previously reported rad51b mutant. The Rad51 paralog mutations all impair HR, as measured by targeted integration and sister chromatid exchange. Remarkably, the mutant cell lines all exhibit very similar phenotypes: spontaneous chromosomal aberrations, high sensitivity to killing by cross-linking agents (mitomycin C and cisplatin), mild sensitivity to gamma rays, and significantly attenuated Rad51 focus formation during recombinational repair after exposure to gamma rays. Moreover, all mutants show partial correction of resistance to DNA damage by overexpression of human Rad51. We conclude that the Rad51 paralogs participate in repair as a functional unit that facilitates the action of Rad51 in HR.

XRCC2 and XRCC3, new human Rad51-family members, promote chromosome stability and protect against DNA cross-links and other damages

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.

Comparative affinity of duloxetine and venlafaxine for serotonin and norepinephrine transporters in vitro and in vivo, human serotonin receptor subtypes, and other neuronal receptors

The blockade of serotonin (5-HT) and norepinephrine (NE) transporters in vitro and in vivo by the dual 5-HT/NE reuptake inhibitors duloxetine and venlafaxine was compared. Duloxetine inhibited binding to the human NE and 5-HT transporters with K(i) values of 7.5 and 0.8 nM, respectively, and with a K(i) ratio of 9. Venlafaxine inhibited binding to the human NE and 5-HT transporters with K(i) values of 2480 and 82 nM, respectively, and with a K(i) ratio of 30. Duloxetine inhibited ex vivo binding to rat 5-HT transporters and NE transporters with ED(50) values of 0.03 and 0.7 mg/kg, respectively, whereas venlafaxine had ED(50) values of 2 and 54 mg/kg, respectively. The depletion of rat brain 5-HT by p-chloramphetamine and depletion of rat hypothalamic NE by 6-hydroxydopamine was blocked by duloxetine with ED(50) values of 2.3 and 12 mg/kg, respectively. Venlafaxine had ED(50) values of 5.9 and 94 mg/kg for blocking p-chloramphetamine- and 6-hydroxydopamine-induced monoamine depletion, respectively. Thus, duloxetine more potently blocks 5-HT and NE transporters in vitro and in vivo than venlafaxine.

Alteration of type A behavior and its effect on cardiac recurrences in post myocardial infarction patients: summary results of the recurrent coronary prevention project

One thousand thirteen post myocardial infarction patients were observed for 4.5 years to determine whether their type A (coronary-prone) behavior could be altered and the effect such alteration might have on the subsequent cardiac morbidity and mortality rates of these individuals. Eight hundred sixty-two of these individuals were randomly assigned either to a control section of 270 participants who received group cardiac counseling or an experimental section of 592 participants who received both group cardiac counseling and type A behavioral counseling. The remaining 151 patients, serving as a "comparison group," did not receive group counseling of any kind. Using the "Intention-to-Treat" principle, we observed markedly reduced type A behavior at the end of 4.5 years in 35.1% of participants given cardiac and type A behavior counseling compared with 9.8% of participants given only cardiac counseling. The cumulative 4.5-year cardiac recurrence rate was 12.9% in the 592 participants in the experimental group that received type A counseling. This recurrence rate was significantly less (p less than 0.005) than either the recurrence rate (21.2%) observed in the 270 participants in the control group or the recurrence rate (28.2%) in those of the comparison group not receiving any special treatment. After the first year, a significant difference in number of cardiac deaths between the experimental and control participants was observed during the remaining 3.5 years of the study. Overall, the results of this study demonstrate for the first time, within a controlled experimental design, that altering type A behavior reduces cardiac morbidity and mortality in post infarction patients.

Homologous recombinational repair of DNA ensures mammalian chromosome stability

The process of homologous recombinational repair (HRR) is a major DNA repair pathway that acts on double-strand breaks and interstrand crosslinks, and probably to a lesser extent on other kinds of DNA damage. HRR provides a mechanism for the error-free removal of damage present in DNA that has replicated (S and G2 phases). Thus, HRR acts in a critical way, in coordination with the S and G2 checkpoint machinery, to eliminate chromosomal breaks before the cell division occurs. Many of the human HRR genes, including five Rad51 paralogs, have been identified, and knockout mutants for most of these genes are available in chicken DT40 cells. In the mouse, most of the knockout mutations cause embryonic lethality. The Brca1 and Brca2 breast cancer susceptibility genes appear to be intimately involved in HRR, but the mechanistic basis is unknown. Biochemical studies with purified proteins and cell extracts, combined with cytological studies of nuclear foci, have begun to establish an outline of the steps in mammalian HRR. This pathway is subject to complex regulatory controls from the checkpoint machinery and other processes, and there is increasing evidence that loss of HRR gene function can contribute to tumor development. This review article is meant to be an update of our previous review [Biochimie 81 (1999) 87].

A CHO-cell strain having hypersensitivity to mutagens, a defect in DNA strand-break repair, and an extraordinary baseline frequency of sister-chromatid exchange

A mutant of CHO cells (strain EM9) previously isolated on the basis of hypersensitivity to killing by ethyl methanesulfonate (EMS) is approx. 10-fold more sensitive than the parental line, AA8, to killing by both EMS and MMS. It is also hypersensitive to killing by other alkylating agents (ethyl nitrosourea and N-methyl-N'-nitro-N-nitrosoguanidine), X-rays, and ultraviolet radiation. The production and repair of DNA single-strand breaks (SSB) were studied using the technique of alkaline elution of DNA from filters. After exposure to 4 Gy of X-rays at 0 degrees C and subsequent incubation at 25 degrees C, SSB were repaired within 12 min in AA8, but little repair occurred in EM9. Similarly, with doses of EMS or MMS that produced comparable numbers of SSB in AA8 and EM9 at the end of a 10-min exposure, repair of SSB occurred more rapidly in AA8 than in EM9, suggesting that individual SSB are longer lived in EM9. EM9 was found to be hypersensitive also to the induction of mutations and sister-chromatid exchanges (SCE) by EMS; per unit dose the mutant had twice as many mutations to thioguanine resistance, 3 times as many mutations to azaadenine resistance, and a 7-fold enhancement in SCE, compared to AA8. Moreover, the baseline frequency of SCE in the mutant was extraordinarily high, i.e., 8.6 +/- 0.6 vs. 107 +/- 5 SCE/cell for AA8 and EM9, respectively, with 10 microM BrdUrd in the medium. The high SCE frequency in EM9 did not vary significantly with BrdUrd concentration over the range examined from 2.5 to 20 microM, and the percentage of 5-bromouracil substitution in the DNA was the same in EM9 and AA8 under these conditions. These data, however, do not rule out the possibility that the high SCE frequency in EM9 is a consequence of an altered sensitivity to incorporated BrdUrd. Thus, EM9 may carry a pleiotropic mutation affecting some function in DNA replication and/or DNA repair and causing the variety of phenotypic properties described in this study.

Human xeroderma pigmentosum group D gene encodes a DNA helicase

Xeroderma pigmentosum (XP), a genetically heterogeneous human disease, results from a defect in nucleotide excision repair of ultraviolet-damaged DNA. XP patients are extremely sensitive to sunlight and suffer from a high incidence of skin cancers. Cell fusion studies have identified seven XP complementation groups, A-G. Group D is of particular interest as mutations in this gene can also cause Cockayne's syndrome and trichothiodystrophy. The XPD gene was initially named ERCC2 (excision repair cross complementing) as it was cloned using human DNA to complement the ultraviolet sensitivity of a rodent cell line. We have purified the XPD protein to near homogeneity and show that it possesses single-stranded DNA-dependent ATPase and DNA helicase activities. We tested whether XPD can substitute for its yeast counterpart RAD3, which is essential for excision repair and for cell viability. Expression of the XPD gene in Saccharomyces cerevisiae can complement the lethality defect of a mutation in the RAD3 gene, suggesting that XPD is an essential gene in humans.

Requirement for the Xrcc1 DNA base excision repair gene during early mouse development

Surveillance and repair of DNA damage are essential for maintaining the integrity of the genetic information that is needed for normal development. Several multienzyme pathways, including the excision repair of damaged or missing bases, carry out DNA repair in mammals. We determined the developmental role of the X-ray cross-complementing (Xrcc)-1 gene, which is central to base excision repair, by generating a targeted mutation in mice. Heterozygous matings produced Xrcc1-/- embryos at early developmental stages, but not Xrcc1-/- late-stage fetuses or pups. Histology showed that mutant (Xrcc1-/-) embryos arrested at embryonic day (E) 6.5 and by E7.5 were morphologically abnormal. The most severe abnormalities observed in mutant embryos were in embryonic tissues, which showed increased cell death in the epiblast and an altered morphology in the visceral embryonic endoderm. Extraembryonic tissues appeared relatively normal at E6.5-7.5. Even without exposure to DNA-damaging agents, mutant embryos showed increased levels of unrepaired DNA strand breaks in the egg cylinder compared with normal embryos. Xrcc1-/- cell lines derived from mutant embryos were hypersensitive to mutagen-induced DNA damage. Xrcc1 mutant embryos that were also made homozygous for a null mutation in Trp53 underwent developmental arrest after only slightly further development, thus revealing a Trp53-independent mechanism of embryo lethality. These results show that an intact base excision repair pathway is essential for normal early postimplantation mouse development and implicate an endogenous source of DNA damage in the lethal phenotype of embryos lacking this repair capacity.

Primary cardiac valve tumors

To investigate the characteristics of primary cardiac valve tumors, we retrospectively analyzed our multiinstitutional experience from 1932 through 1990. We encountered 56 valvular tumors in 53 patients. The average age of these patients was 52 years (range, 2 to 88 years) and 79% (42/53) were male. Symptoms were present in 38% (20/53) and were neurological in 15% (8/53). Four patients experienced sudden death. Each of the four valves was affected with approximately equal frequency: 16 aortic, 15 mitral, 13 pulmonary, and 12 tricuspid. All but four tumors were benign. The most common histological type was papillary fibroelastoma (41), followed by myxomas (5), fibromas (4), sarcomas (2), hamartoma (1), hemangioma (1), histiocytoma (1), and undifferentiated (1). Average tumor size was 1.15 cm (range, 3 mm to 7 cm), and the average size of fibroelastomas was 8 mm (range, 3 to 15 mm). Mitral valve tumors were more likely than aortic valve tumors to produce serious neurological symptoms or sudden death (8/15 versus 3/16; p less than 0.05). Six patients underwent echocardiography, and results were positive in each. All 6 underwent uncomplicated valve repair or replacement. Compared with a series of 407 nonvalvular tumors, cardiac valve tumors are more likely to occur in male patients (p less than 0.001) and adults (p less than 0.001). Valve tumors are also more commonly benign (p less than 0.001) and asymptomatic (p less than 0.001). These tumors demonstrate somewhat less aggressive behavior compared with non-valvular tumors, but their distinct propensity to produce serious clinical sequelae argues in favor of surgical resection for all cardiac valve tumors.

Monoclonal origins of malignant mixed tumors (carcinosarcomas). Evidence for a divergent histogenesis

Malignant mixed tumors (carcinosarcomas) are examples of unusual neoplasms whose occurrences have been observed in increasingly diverse sites but whose pathogenesis remains a complete mystery. Two antithetical hypotheses that have been advanced to explain the histogenesis of these tumors include the convergence hypothesis, which proposes an origin from two or more stem cells (multiclonal hypothesis), and the divergence hypothesis, which proposes an origin from a single totipotential stem cell that differentiates into separate epithelial and mesenchymal directions (monoclonal hypothesis). To test these hypotheses, a novel strategy for the determination of clonality from as few as 100 tumor cells obtained by enzymatic digestion of either fresh or formalin-fixed, paraffin-embedded tissues and cell sorting was used that exhibited the polymerase chain reaction (PCR) in amplifying a 511-bp region located within the first intron of the human hypoxanthine phosphoribosyl transferase gene, a site that contains inactive X chromosomal obligately methylated HpaII/MspI sites and single-base allelic polymorphisms in 5% females. Carcinoma cells gated on the basis of fluorescein isothiocyanate (FITC)-anti-cytokeratin and sarcoma cells gated on the basis of FITC-antivimentin or FITC-anti-desmin were sorted to homogeneity on FACSTAR and then subjected to genomic DNA extraction and Hpa II digestion before PCR amplification and subsequent analysis of the product on denaturing gradient gel electrophoresis. The comigrations of the single homoduplexes generated from both the carcinoma cells and sarcoma cells in six different malignant mixed tumors obtained from four different organs indicated clonal identity and monoclonality in all cases. These findings of monoclonality were confirmed independently by two other methods of clonality determination. The findings of a monoclonal origin of carcinosarcomas support the single totipotential stem-cell-divergence hypothesis.

Characterization of the XRCC1-DNA ligase III complex in vitro and its absence from mutant hamster cells

The human DNA repair protein XRCC1 was overexpressed as a histidine-tagged polypeptide (denoted XRCC1-His) in Escherichia coli and purified in milligram quantities by affinity chromatography. XRCC1-His complemented the mutant Chinese hamster ovary cell line EM9 when constitutively expressed from a plasmid or when introduced by electroporation. XRCC1-His directly interacted with human DNA ligase III in vitro to form a complex that was resistant to 2 M NaCl. XRCC1-His interacted equally well with DNA ligase III from Bloom syndrome, HeLa and MRC5 cells, indicating that Bloom syndrome DNA ligase III is normal in this respect. Detection of DNA ligase III on far Western blots by radiolabelled XRCC1-His indicated that the level of the DNA ligase polypeptide was reduced approximately 4-fold in the mutant EM9 and also in EM-C11, a second member of the XRCC1 complementation group. Decreased levels of polypeptide thus account for most of the approximately 6-fold reduced DNA ligase III activity observed previously in EM9. Immunodetection of XRCC1 on Western blots revealed that the level of this polypeptide was also decreased in EM9 and EM-C11 (> 10-fold), indicating that the XRCC1-DNA ligase III complex is much reduced in the two CHO mutants.

A screening method for isolating DNA repair-deficient mutants of CHO cells

A simple procedure for isolating mutagen-sensitive clones of CHO cells was developed and applied in mutant hunts in which colonies were screened for hypersensitivity to killing by ultraviolet radiation (UV, ethyl methanesulfonate (EMS), or mitomycin C (MMC). Each of two UV-sensitive clones studied in detail had a D37 dose of 1.0 J/m2 compared to 7.0 J/m2 for the wild-type cells, and each was shown to have no detectable repair replication following exposure to UV doses of up to 26 J/m2. Although these mutants resemble xeroderma pigmentosum human mutants with respect to their repair defect and cross-sensitivity to the carcinogen 4-nitroquinoline-1-oxide, one of two clones (UV-20) is characterized by extreme hypersensitivity to MMC (80-fold as compared to the wild type). Clones having hypersensitivity to alkylating agents, but not UV, were obtained using MMC and EMS. In the latter case the two clones had significantly increased sensitivity to the killing action of 60Co gamma-rays.

The Rad51 paralog Rad51B promotes homologous recombinational repair

The highly conserved Saccharomyces cerevisiae Rad51 protein plays a central role in both mitotic and meiotic homologous DNA recombination. Seven members of the Rad51 family have been identified in vertebrate cells, including Rad51, Dmc1, and five Rad51-related proteins referred to as Rad51 paralogs, which share 20 to 30% sequence identity with Rad51. In chicken B lymphocyte DT40 cells, we generated a mutant with RAD51B/RAD51L1, a member of the Rad51 family, knocked out. RAD51B(-/-) cells are viable, although spontaneous chromosomal aberrations kill about 20% of the cells in each cell cycle. Rad51B deficiency impairs homologous recombinational repair (HRR), as measured by targeted integration, sister chromatid exchange, and intragenic recombination at the immunoglobulin locus. RAD51B(-/-) cells are quite sensitive to the cross-linking agents cisplatin and mitomycin C and mildly sensitive to gamma-rays. The formation of damage-induced Rad51 nuclear foci is much reduced in RAD51B(-/-) cells, suggesting that Rad51B promotes the assembly of Rad51 nucleoprotein filaments during HRR. These findings show that Rad51B is important for repairing various types of DNA lesions and maintaining chromosome integrity.

Correction of chromosomal instability and sensitivity to diverse mutagens by a cloned cDNA of the XRCC3 DNA repair gene

The mutagen-sensitive CHO line irs1SF was previously isolated on the basis of hypersensitivity to ionizing radiation and was found to be chromosomally unstable as well as cross-sensitive to diverse kinds of DNA-damaging agents. The analysis of somatic cell hybrids formed between irs1SF and human lymphocytes implicated a human gene (defined as XRCC3; x-ray repair cross-complementing), which partially restored mitomycin C resistance to the mutant. A functional cDNA that confers mitomycin C resistance was transferred to irs1SF cells by transforming them with an expression cDNA library and obtaining primary and secondary transformants. Functional cDNA clones were recovered from a cosmid library prepared from a secondary transformant. Transformants also showed partial correction of sensitivity to cisplatin and gamma-rays, efficient correction of chromosomal instability, and substantially improved plating efficiency and growth rate. The XRCC3 cDNA insert is approximately 2.5 kb and detects an approximately 3.0-kb mRNA on Northern blots. The cDNA was mapped by fluorescence in situ hybridization to human chromosome 14q32.3, which was consistent with the chromosome concordance data of two independent hybrid clone panels.

Effect of melatonin on jet lag after long haul flights

OBJECTIVE

To determine whether doses of the pineal hormone melatonin alleviate jet lag.

DESIGN

Double blind, placebo controlled crossover trial.

SETTING

Long haul return flights from Auckland, New Zealand, to London and back.

SUBJECTS

Twenty volunteers with experience of transcontinental flights (eight women and 12 men aged 28 to 68).

INTERVENTIONS

Melatonin (or placebo) 5 mg three days before flight, during flight, and once a day for three days after arrival.

END POINT

Symptoms of jet lag.

MEASUREMENTS AND MAIN RESULTS

Visual analogue scale for feelings of jet lag and tiredness; profile of moods states questionnaire for vigour-activity and fatigue-inertia; and retrospective ratings 10 days after arrival of sleep pattern, energy, and daytime tiredness. Feelings of jet lag were less for subjects taking melatonin (mean score 2.15 v 3.4); these subjects took fewer days than the placebo group to establish a normal sleep pattern (2.85 v 4.15), to not feel tired during the day (3.0 v 4.6), and to reach normal energy levels (3.25 v 4.7). Results for fatigue-inertia and vigour-activity were similar. For all subjects jet lag was more severe on the return (westward) than the outward (eastward) journey.

CONCLUSIONS

Melatonin can alleviate jet lag and tiredness after long haul flights.

A summary of mutations in the UV-sensitive disorders: xeroderma pigmentosum, Cockayne syndrome, and trichothiodystrophy

The human diseases xeroderma pigmentosum, Cockayne syndrome, and trichothiodystrophy are caused by mutations in a set of interacting gene products, which carry out the process of nucleotide excision repair. The majority of the genes have now been cloned and many mutations in the genes identified. The relationships between the distribution of mutations in the genes and the clinical presentations can be used for diagnosis and for understanding the functions and the modes of interaction among the gene products. The summary presented here represents currently known mutations that can be used as the basis for future studies of the structure, function, and biochemical properties of the proteins involved in this set of complex disorders, and may allow determination of the critical sites for mutations leading to different clinical manifestations. The summary indicates where more data are needed for some complementation groups that have few reported mutations, and for the groups for which the gene(s) are not yet cloned. These include the Xeroderma pigmentosum (XP) variant, the trichothiodystrophy group A (TTDA), and ultraviolet sensitive syndrome (UVs) groups. We also recommend that the XP-group E should be defined explicitly through molecular terms, because assignment by complementation in culture has been difficult. XP-E by this definition contains only those cell lines and patients that have mutations in the small subunit, DDB2, of a damage-specific DNA binding protein.

Alteration of type A behavior and reduction in cardiac recurrences in postmyocardial infarction patients

Eight hundred sixty-two postmyocardial infarction patients volunteered to be randomly selected and enrolled into: (1) a control section of 270 patients, who received group cardiologic counseling; and (2) an experimental section of 592 patients, who received group type A behavior counseling in addition to group cardiologic counseling. Reduction in type A behavior at the end of 3 years was observed in 43.8% of the 592 participants, who initially were enrolled to receive group cardiologic and type A behavioral counseling. This degree of behavioral reduction was significantly greater than that observed in participants who initially were enrolled to receive only group cardiologic counseling. The 3-year cumulative cardiac recurrence rate was 7.2% in participants who initially were enrolled to receive group cardiologic and type A behavioral counseling. This was significantly less (p less than 0.005) than that (13%) observed in participants who initially were enrolled to receive only cardiologic counseling. This difference in recurrence rates was due to a lesser incidence of nonfatal infarctions in the patients who had been enrolled in the section receiving type A behavioral as well as cardiologic counseling. These data suggest that type A behavior can be altered in a sizable fraction of postinfarction patients and that such alteration is associated with a significantly reduced rate of nonfatal myocardial infarctions.

ERCC2: cDNA cloning and molecular characterization of a human nucleotide excision repair gene with high homology to yeast RAD3

Human ERCC2 genomic clones give efficient, stable correction of the nucleotide excision repair defect in UV5 Chinese hamster ovary cells. One clone having a breakpoint just 5' of classical promoter elements corrects only transiently, implicating further flanking sequences in stable gene expression. The nucleotide sequences of a cDNA clone and genomic flanking regions were determined. The ERCC2 translated amino acid sequence has 52% identity (73% homology) with the yeast nucleotide excision repair protein RAD3. RAD3 is essential for cell viability and encodes a protein that is a single-stranded DNA dependent ATPase and an ATP dependent helicase. The similarity of ERCC2 and RAD3 suggests a role for ERCC2 in both cell viability and DNA repair and provides the first insight into the biochemical function of a mammalian nucleotide excision repair gene.

Socially Shared Cognition, Affect, and Behavior: A Review and Integration

In this article, we review 4 classes of models of socially shared cognition and behavior: supraindividual models, information-processing models, communication models, and social interaction models. Our review draws on research and theory in social psychology, sociology, and organization behavior. We conclude that these innovative perspectives on socially shared behavior represent a new approach to the study of groups and are distinct from traditional models of the group mind and crowd behavior. The key processes implicated in these models focus on the potency of immediate interaction, reciprocal influence processes between individuals and groups, goal-directed behavior, negotiated processing of information and ideas, and the maintenance and enhancement of social identity. This approach to socially shared understanding is not antagonistic toward the analysis of individual-level processes but rather maintains that individual-level processes are necessary but not sufficient to build a social psychology of shared understanding.