DNA ligase IV from HeLa cell nuclei

A human cDNA encoding a previously unrecognized DNA ligase IV has been identified (Wei, Y.-F., Robins, P., Carter, K., Caldecott, K., Pappin, D. J. C., Yu, G.-L., Wang, R.-P., Shell, B. K., Nash, R. A., Schär, P., Barnes, D. E., Haseltine, W. A., and Lindahl, T. (1995) Mol. Cell. Biol. 15, 3206-3216). Antibodies have been raised against predicted peptide sequences of DNA ligase IV and used to identify the enzyme during purification from HeLa cell nuclei. The 96-kDa DNA ligase IV and the 103-kDa DNA ligase III co-migrate during SDS-polyacrylamide gel electrophoresis and have similar column fractionation properties, which complicates the distinction between the two enzymes, but they have been separated by Mono S liquid chromatography. During initial size fractionation by gel chromatography in 1 M NaCl, DNA ligase IV elutes in the same position as the DNA ligase III-XRCC1 protein complex, indicating that DNA ligase IV is also bound to another protein or occurs as a dimer. DNA ligase IV has been purified free from other DNA ligases, and its enzymatic properties have been examined. The purified protein effectively joins single-strand breaks in a double-stranded polydeoxynucleotide in an ATP-dependent reaction. The substrate specificity of DNA ligase IV differs from those of the other two cloned human DNA ligases, I and III, with regard to their ability to join the hybrid substrates oligo(dT).poly(rA) and oligo(rA).poly(dT). DNA ligase IV occurs in part as an enzyme-adenylate complex in HeLa cell nuclear extracts.

Violence and remote area nursing

The Federal government and others have accepted that violence is a problem in rural and remote communities across Australia. The issue of personal safety for Remote Area Nurses (RANs) living in remote areas has been mentioned in a number of State and Regional reports. However, to the knowledge of this research group, no systematic attempt to gather data on the incidence of violence experienced by RANs and other remote health workers has occurred. This study aims to redress the lack of data on the experience and description of violence against remote area nurses. Results indicate that remote area nurses are living with frequent threats to their personal safety while on duty, on call and off duty, and that violent incidents are often handled badly by employers, the community and remote area nurses themselves. Twenty-four-hour call is an employment requirement for 82% of respondents. Those required to be on call 24 hours experience episodes of violence more frequently. Despite nearly all of the respondents experiencing episodes of violence within the 12 months prior to the study, there was a tendency for remote area nurses, except in the case of physical violence, to perceive both the frequency and the severity of their experiences with violence as low and not personally directed.

Molecular cloning and expression of human cDNAs encoding a novel DNA ligase IV and DNA ligase III, an enzyme active in DNA repair and recombination

Three distinct DNA ligases, I to III, have been found previously in mammalian cells, but a cloned cDNA has been identified only for DNA ligase I, an essential enzyme active in DNA replication. A short peptide sequence conserved close to the C terminus of all known eukaryotic DNA ligases was used to search for additional homologous sequences in human cDNA libraries. Two different incomplete cDNA clones that showed partial homology to the conserved peptide were identified. Full-length cDNAs were obtained and expressed by in vitro transcription and translation. The 103-kDa product of one cDNA clone formed a characteristic complex with the XRCC1 DNA repair protein and was identical with the previously described DNA ligase III. DNA ligase III appears closely related to the smaller DNA ligase II. The 96-kDa in vitro translation product of the second cDNA clone was also shown to be an ATP-dependent DNA ligase. A fourth DNA ligase (DNA ligase IV) has been purified from human cells and shown to be identical to the 96-kDa DNA ligase by unique agreement between mass spectrometry data on tryptic peptides from the purified enzyme and the predicted open reading frame of the cloned cDNA. The amino acid sequences of DNA ligases III and IV share a related active-site motif and several short regions of homology with DNA ligase I, other DNA ligases, and RNA capping enzymes. DNA ligases III and IV are encoded by distinct genes located on human chromosomes 17q11.2-12 and 13q33-34, respectively.

Structural and functional homology between mammalian DNase IV and the 5'-nuclease domain of Escherichia coli DNA polymerase I

A nuclear 42-kDa 5'-->3'-exonuclease, DNase IV, was found previously in animal tissues. The enzyme has been purified from HeLa cells and shown to possess two catalytic properties characteristic of the 5'-nuclease function of Escherichia coli DNA polymerase I,-DNase IV removes single-stranded 5' regions from splayed-arm DNA structures by endonucleolytic incision at the bifurcation point and possesses RNase H activity. Determination of the molecular masses of tryptic and V8 peptides of DNase IV by mass spectrometry identified the enzyme as the human homolog of the Schizosaccharomyces pombe Rad2 protein. The protein sequence retains conserved residues and shows significant homology to the sequences of the 5'-nuclease domain of E. coli DNA polymerase I and related microbial enzymes.

Different active sites of mammalian DNA ligases I and II

Bovine DNA ligases I and II were adenylylated in the presence of [alpha-32P]ATP and digested with limiting amounts of trypsin or V8 protease. The generation of radioactive peptides of decreasing size was monitored by polyacrylamide gel electrophoresis and autoradiography. Active site peptides obtained by complete proteolytic digestions with trypsin, V8, or Lys-C protease were also compared. The partial digestion products of DNA ligases I and II were entirely different, with no indication of extensive sequence homology. Furthermore, the sequence of the active site region of DNA ligase I is clearly different from that of DNA ligase II. Similar analysis of a third chromatographically distinct mammalian DNA ligase indicated that it is different from DNA ligase I but related to DNA ligase II.

Complementation of DNA repair in xeroderma pigmentosum group A cell extracts by a protein with affinity for damaged DNA

Complementation group A of xeroderma pigmentosum (XP) represents one of the most prevalent and serious forms of this cancer-prone disorder. Because of a marked defect in DNA excision repair, cells from individuals with XP-A are hypersensitive to the toxic and mutagenic effects of ultraviolet light and many chemical agents. We report here the isolation of the XP-A DNA repair protein by complementation of cell extracts from a repair-defective human XP-A cell line. XP-A protein purified from calf thymus migrates on denaturing gel electrophoresis as a doublet of 40 and 42 kilodaltons. The XP-A protein binds preferentially to ultraviolet light-irradiated DNA, with a preference for damaged over nondamaged nucleotides of approximately 10(3). This strongly suggests that the XP-A protein plays a direct role in the recognition of and incision at lesions in DNA. We further show that this protein corresponds to the product encoded by a recently isolated gene that can restore excision repair to XP-A cells. Thus, excision repair of plasmid DNA by cell extracts sufficiently resembles genomic repair in cells to reveal accurately the repair defect in an inherited disease. The general approach described here can be extended to the identification and isolation of other human DNA repair proteins.

[Moh's micrographic surgery: first experience in Israel]

Moh's micrographic surgery is a method by which cancer of the skin may be excised under complete microscopic control. It consistently offers the highest cure rate in the treatment of malignancy of the skin, while the least amount of uninvolved tissue is sacrificed. This, the first center for Moh's micrographic surgery in Israel, started functioning in 1989. The first 50 patients included 33 women (average age 50.7 years) and 17 men (average age 60.6 years) suffering from basal cell carcinoma of the face. 56% of the patients were referred for Moh's surgery after failure of other primary treatments, and the rest were referred due to the problematic location of the lesion or its size. 13 patients (28%) were referred after previous treatment with solcoderm (Mardi's method). In these patients the lesions were especially deep and aggressive.

Prolongation of anesthesia in Mohs micrographic surgery with 2% lidocaine jelly

The authors performed a prospective, randomized trial of topical 2% lidocaine jelly in patients with basal cell or squamous cell carcinomas to test the effectiveness of the jelly in prolonging the time of anesthesia between the multiple stages of Mohs micrographic surgery. There was a 48% increase in the duration of anesthesia achieved by the use of 2% lidocaine jelly in patients given 1% lidocaine with epinephrine. In patients given 1% lidocaine without epinephrine, there was approximately a 2.5 times greater duration of anesthesia achieved by using topical 2% lidocaine jelly.

Effect of exogenous DNA fragments on human cell extract-mediated DNA repair synthesis

Extracts from HeLa cells were used to study the susceptibility of repair synthesis in UV-irradiated plasmid DNA to inhibition by exogenously added nucleic acid. Purified DNA restriction fragments have little inhibitory effect on repair synthesis. However, activated calf thymus DNA fragments, genomic DNA fragments in cell extracts, and sonicated plasmid DNA all inhibited repair synthesis. Degraded DNA fragments arising from E. coli during bacterial plasmid purification were found to be particularly inhibitory. tRNA is not a potent inhibitor of in vitro repair synthesis. In order to observe efficient DNA repair synthesis mediated by human cell extracts, it is essential to prepare highly purified closed circular plasmid DNA, and we describe a reliable method for doing so.

Use of in vivo and in vitro assays for the characterization of mammalian excision repair and isolation of repair proteins

Elucidation of the molecular mechanism of mammalian nucleotide excision repair requires the availability of purified proteins, DNA substrates with defined lesions and suitable repair assays. Repair assays introduced in recent years vary from testing individual steps and successions of steps in vitro to systems that closely reflect the entire process in vivo. In the first part of this review, an in vivo microinjection system is discussed. The second part of the article reviews an in vitro system for study of repair synthesis promoted by cell extracts. Both systems can be utilized as assays during the purification of protein factors that complement repair-defective xeroderma pigmentosum cells. The effect of purified repair proteins from other organisms on mammalian repair is also considered.

DNA repair replication by soluble extracts from human lymphoid cell lines

A system is described in which extracts from human cells can perform repair replication on DNA damaged by ultraviolet light or chemical carcinogens. Whole cell extracts from lymphoid cell lines are incubated with damaged plasmid DNA circles at 30 degrees C in the presence of ATP and the four deoxynucleoside triphosphates. Repair synthesis is monitored by the incorporation of alpha-32P-dATP into closed circular plasmid molecules. Analysis of the time course of the reaction suggests that the slowest step in repair is incision, rather than polymerization or ligation. The size of repair patches inserted into ultraviolet-irradiated DNA during a reaction was estimated by substitution of thymidine triphosphate with 5-bromodeoxyuridine triphosphate and sedimentation in alkaline cesium chloride gradients. Patches with heterogeneous sizes of less than 120 bases were observed.

Complementation of the xeroderma pigmentosum DNA repair defect in cell-free extracts

Soluble extracts from human lymphoid cell lines that perform repair synthesis on covalently closed circular DNA containing pyrimidine dimers or psoralen adducts are described. Short patches of nucleotides are introduced by excision repair of damaged DNA in an ATP-dependent reaction. Extracts from xeroderma pigmentosum cell lines fail to act on damaged circular DNA, but are proficient in repair synthesis of ultraviolet-irradiated DNA containing incisions generated by Micrococcus luteus pyrimidine dimer-DNA glycosylase. Repair is defective in extracts from all xeroderma pigmentosum cell lines investigated, representing the genetic complementation groups A, B, C, D, H, and V. Mixing of cell extracts of group A and C origin leads to reconstitution of the DNA repair activity.

Activity of a new nitrosourea (TCNU) in human lung cancer xenografts

The activity of a new nitrosourea (TCNU) based on the endogenous amino acid taurine was assessed in three human lung cancer xenografts growing in immunodeficient mice. Moderate activity (specific growth delays of 0.63 and 1.13 compared with controls) was seen in two non-small cell tumours after a single oral administration of 20 mg-1kg. This dose was curative in a small cell xenograft. By using high performance liquid chromatography it was possible to detect parent drug in the tumours as well as the plasma and tissues after oral administration of TCNU. Drug sensitivity was correlated inversely with the amount of the DNA repair enzyme 0(6)-methylguanine-DNA methyltransferase assayed from extracts of the tumour cells but not with the levels of parent drug within the tumour. This compound appears to have unique pharmacokinetic properties compared with other chloroethylnitrosoureas.

Functional domains and methyl acceptor sites of the Escherichia coli ada protein

The ada gene of Escherichia coli encodes a 39-kDa protein which serves both as a transcriptional activator of the adaptive response to alkylating agents and as a DNA repair enzyme demethylating O6-methyl-guanine and phosphotriester residues. Here, the isolated Ada protein was found to be readily cleaved into two fragments of similar size by treatment with trypsin, chymotrypsin, subtilisin, or V8 protease. The fragments retained their respective methyltransferase activities. The Ada protein is, therefore, comprised of two stable active domains united by a central hinge region of about 10 amino acids. Post-translational modification of the Ada protein by methylation of a specific cysteine residue in the NH2-terminal domain is known to convert it to an efficient transcriptional activator. This residue has now been identified as Cys-69.