Supercoils in human DNA

Detection of carcinogen-induced DNA breaks by nick translation in permeable cells

A nick-translation reaction with E. coli DNA polymerase I (pol. I) was used to detect in situ DNA breaks produced by chemical carcinogens. Normal human fibroblasts treated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in various doses were permeabilized with lysolecithin, and were nick translated in the presence of [3H]dCTP and pol. I. The radioactivity incorporated increased with MNNG concentration, and was directly proportional to the poly(ADP-ribose) synthetase activity. Other DNA-damaging agents such as bleomycin or 4-nitroquinoline 1-oxide also caused the nick translation rate to increase. When MNNG-treated cells were cultured in fresh medium containing no MNNG, the increase in the rate of nick translation in permeable cells became less and this decrease was abolished by addition of aphidicolin or cytosine arabinoside. The nick translation method described here may be a useful means for estimating intrinsic DNA breaks in cells treated with carcinogens.

Chinese hamster metaphase chromosomes isolated under physiological conditions. A partial characterization of associated non-histone proteins and protein cores

In a previous report [2] we have described a non-histone protein core which could be isolated from Chinese hamster metaphase chromosomes. This core structure maintained the overall morphology of the metaphase chromosome even after removal of all of the histones, together with many of the non-histone proteins and the bulk of the DNA. As part of our work on the characterization of these core structures, we have developed a novel procedure for the isolation of metaphase chromosomes which avoids the use of high pH buffers and hexylene glycol, as well as eliminating the numerous centrifugation and resuspension steps previously employed. Chromosome cores prepared by 2 M NaCl extraction and DNase I digestion from metaphase chromosomes isolated under these more gentle, quasi-physiological conditions, are shown to contain a relatively simple subset of non-histone proteins. One-dimensional SDS-polyacrylamide gel electrophoresis shows two major groups of polypeptides having molecular weights 48 000-52 000 and 65 000-72 000 D respectively, with similarities in mobilities to the nuclear pore complex-lamina polypeptides and tubulins. However, more detailed analysis by two-dimensional gel electrophoresis and peptide mapping has failed to detect these proteins. A 52 000 D polypeptide component of the core is tentatively identified as the intermediate filament protein vimentin. The in vivo significance of chromosome cores is discussed.

Association of thyroid hormone receptors with chromatin

A large body of circumstantial evidence indicates that receptors located in nuclei of T3 responsive tissues represent a site of initiation of thyroid hormone action at the cellular level. Partial characterization of T3 receptors indicates that these proteins are monomeric structures in nuclei and are chromatin-associated non-histone proteins. Treatment of rat liver nuclei with either pancreatic DNase I or micrococcal nuclease releases T3 receptors from nuclei in two forms: a predominant (95 400 Mr; 5.5-6.0S) and a minor (265 000-365 000 Mr; 12.5S) nucleoprotein complex. Similar structures are excised from rat kidney, brain, and heart nuclei and from GH1 pituitary cell nuclei by micrococcal nuclease digestion. These endonuclease-excised receptor-containing complexes are significantly larger than the salt-extracted receptor (50 000 Mr; 3.5S). The presence of DNA and other non-receptor proteins in these structures indicates that T3 receptors probably function within multimeric complexes in vivo. Although T3 receptors appear to be associated with DNA between nucleosomes, i.e. linker DNA, it is not entirely clear whether all or only a fraction of T3 receptors interact with nucleosomal components. The 12.5S receptor-containing nucleoprotein complex may represent T3 receptors in association with linker DNA and nucleosomal components. T3 receptors do not appear to be uniformly distributed to all chromatin fractions, but are associated with structures having characteristics of transcriptionally active chromatin. They are found in a region of chromatin which is enriched in RNA polymerase activity, rapidly labeled RNA and non-histone proteins, and depleted of histone Hl. This region is also highly sensitive to both micrococcal nuclease and pancreatic DNase I digestion. The association of receptors with transcriptionally active chromatin, however, must be considered provisional until additional details of the precise receptor-chromatin interaction have been established. The recent demonstration of a 20-fold increase in a specific hepatic mRNA four hours following administration of T3 to hypothyroid rats indicates that thyroid hormone potentially has very rapid effects on hepatic gene expression. However, significant changes in nuclear protein phosphorylation, nuclear protein composition, and chromatin structure have not been detected within this four-hour period. Thus, effects of T3 on hepatic gene expression are brought about by local and presumably subtle changes in nuclear function.

An investigation of the genetic toxicology of irradiated foodstuffs using short-term test systems. III--In vivo tests in small rodents and in Drosophila melanogaster

Six in vivo genetic toxicity tests were carried out on irradiated or unirradiated cooked chicken, dried dates and cooked fish. The tests were as follows: sex-linked recessive lethal mutations in Drosophila melanogaster (dried dates only), chromosome aberrations in bone marrow of Chinese hamsters, micronucleus test in rats, mice and Chinese hamsters, sister-chromatid exchange in bone marrow of mice and Chinese hamsters and in spermatogonia of mice, and DNA metabolism in spleen cells of Chinese hamsters. None of the tests provided any evidence of genetic toxicity induced by irradiation. However, dried dates, whether irradiated or not, showed evidence of some genetic toxicity in their effect on DNA metabolism in spleen cells and SCE induction in bone marrow. Feeding irradiated fish affected DNA metabolism in the spleen cells of Chinese hamsters. This effect could be interpreted as an induction of an immunoactive compound, although it could also be explained by the persistence of an immunoactive compound due to the removal by irradiation of spoilage organisms that would normally degrade it.

Adenovirus DNA is associated with the nuclear matrix of infected cells

Viral DNA was found to be tightly associated with the nuclear matrix from HeLa cells lytically infected with human adenovirus type 5. The bound viral DNA, like cell DNA, was resistant to nonionic detergent and to extraction with high-salt (2 M NaCl) solution. However, whereas over 95% of the cell DNA was recovered in the matrix fraction, the amount of associated viral DNA varied during infection. Throughout the lytic cycle, the amount of matrix-associated adenovirus type 5 DNA increased until it reached a plateau level at 20 to 24 h after infection. At this stage, the matrix-bound DNA represented 87% of the total viral DNA; after this stage, additional newly synthesized viral DNA accumulated as non-matrix-associated DNA. DNase digestion studies revealed that all viral DNA sequences were equally represented in the matrix-bound DNA both early and late in infection; thus, unlike cell DNA, there seem to be no preferred attachment sites on the viral genome. An enrichment of viral DNA relative to cell DNA was found in the matrix-associated DNA after extensive DNase I digestion. This finding, together with an in situ hybridization study, suggests that the viral DNA is more intimately associated with the nuclear matrix than is cell DNA and probably does not exist in extended loops.

Repair of gamma-ray induced single-strand breaks in human lymphocytes

Human peripheral blood lymphocytes are one of the most radiosensitive mammalian cells. We tested whether this radiosensitivity was associated with the persistance of unrepaired DNA damage in gamma-irradiated lymphocytes. Repair was determined by measuring the loss and recovery of DNA supercoiling when nucleoids are centrifuged in the presence of increasing amounts of ethidium bromide. Results show a limited repair capacity in resting human lymphocytes after gamma-irradiation.

Supercoiled DNA repair in thymocyte fractions differing in radiosensitivity

Mouse thymocyte fractions were isolated on the basis of buoyant density in a step-gradient of human serum albumin. These fractions were characterized by cell size, radiosensitivity and hydrocortisone sensitivity, and by labelled precursor incorporation into DNA, RNA and proteins. The relative sedimentation of nucleoids in sucrose gradient was also determined. Large and more radioresistant thymocytes were characterized by an increased rate of DNA repair compared with the fraction of radiosensitive small lymphocytes. Nucleoids prepared from the small thymocyte fraction had greater relative sedimentation rates than those derived from large cells. The response of relative sedimentation of nucleoids from large and small thymocyte fractions to ethidium bromide concentration does not permit a conclusion on different superhelix density for DNA in cells of these fractions. At the same time the estimate of the supercoiled domain size for nucleoids of large and small thymocyte fractions showed that in the more radiosensitive small cells domains of greater size were predominant.

DNA repair assays as tests for environmental mutagens. A report of the U.S. EPA Gene-Tox Program

A literature review was undertaken to determine the usefulness of DNA repair assays, other than unscheduled DNA synthesis, as screening techniques for mutagenic carcinogens. 92 reports were found to contain useful data for 49 chemicals using 6 techniques, namely, (1) cesium chloride equilibrium density gradients to study repair replication, (2) benzoylated naphthoylated diethylaminoethyl cellulose columns to study repair replication, (3) 313-nm irradiation of DNA containing bromodeoxyuridine to study repair replication, (4) alkaline elution to study repair of single-strand breaks and crosslinks, (5) alkaline sucrose gradients to study repair of single-strand breaks, and (6) direct assays for removal of adducts from DNA. Almost all of the 49 chemicals studied were known mutagens or carcinogens and/or known inducers of DNA repair, 9 compounds failed to elicit DNA repair by at least 1 assay technique, and at least 3 of these were not tested by the most appropriate and sensitive method. Nevertheless, although valid for studying repair phenomena in eukaryotic cells, these assays are not considered useful for screening. They are time-consuming, expensive, and/or require highly specialized skills. Despite the high frequency of positive reports, it is obvious from the literature that repair assays will fail to detect, or will detect with low efficiency, those agents whose main action is either intercalation or induction of strand breaks. For these and other reasons, DNA repair as a basis for screening for mutagenic carcinogens is not considered to be a useful concept.

Inhibition of DNA synthesis and alteration to DNA structure by the phenacetin analog p-aminophenol

p-Aminophenol a structural analog and minor metabolite of phenacetin has previously been shown to be a potent nephrotoxic agent. In this report we have shown that p-aminophenol has a marked effect on DNA function and structure. DNA synthesis was inhibited in a dose-dependent manner in human lymphoblastoid cells after exposure to p-aminophenol. Results suggest that DNA synthesis is inhibited by the action of p-aminophenol on DNA structure. At low concentrations of p-aminophenol a reduction in the degree of supercoiling of cellular DNA is observed, as determined by sedimentation under neutral conditions. However at higher concentrations an increase in sedimentation of nucleoids (supercoiled molecules) is obtained which is indicative of an increased level of supercoiling or a more compact structural form of DNA due to folding or aggregation. The number of single strand breaks in DNA, when determined by sedimentation in alkaline sucrose gradients, increases with increasing dose of p-aminophenol. The increase in strand breakage observed at lower concentrations of p-aminophenol agrees with the reduced sedimentation rate obtained under neutral conditions. At higher concentrations of p-aminophenol the extent of breakage of DNA increases under alkaline conditions but an increase in sedimentation occurs under neutral conditions.

The ovalbumin gene is associated with the nuclear matrix of chicken oviduct cells

The DNA in a eucaryotic nucleus is arranged into a series of supercoiled loops that are anchored at their bases to the nuclear matrix. Using nuclease digestion, one can progressively cleave DNA from the loops, thereby isolating residual DNA that is progressively closer to the nuclear matrix anchorage sites. We have determined that the ovalbumin gene is preferentially associated with the nuclear matrix of chicken oviduct cells, but is not preferentially associated with the nuclear matrix of chicken liver cells. As a control, the beta-globin gene, which is not transcribed in oviduct cells, was found not to be preferentially associated with the oviduct nuclear matrix. The observation that the transcriptionally active ovalbumin gene is preferentially associated with the nuclear matrix may have significant implications for gene expression and the organization of nuclear DNA into supercoiled-loop domains.

The effect of inhibition of (ADP-ribose)n biosynthesis on DNA repair assayed by the nucleoid technique

DNA damage and repair was assayed by the loss and restoration of DNA supercoiling in nucleoids. This technique was used to assess the effects of inhibition of (ADP-ribose)n biosynthesis by 3-aminobenzamide on the capacity to repair DNA of mouse leukaemia L1210 cells following damage by gamma-irradiation and by the monofunctional alkylating agent, dimethyl sulphate. 3-Aminobenzamide partially inhibits restoration of supercoiling following gamma-irradiation and dimethyl sulphate treatment, but inhibits neither the enzymic incision events leading to breaks in the DNA nor the repair synthesis. This inhibition of repair can be observed at very low doses of damaging agents. These observations confirm and extend the evidence that (ADP-ribose)n biosynthesis is required for efficient cellular recovery from DNA damage. In particular, the nucleoid technique permits the demonstration that 3-aminobenzamide inhibits DNA repair after gamma-radiation; it is not possible to draw this unequivocal conclusion with the data from alkaline sucrose gradients because this technique is too insensitive. 1-beta-D-Arabinofuranosylcytosine, which inhibits semiconservative DNA replication, also retards repair, and this effect can be reversed by the addition of deoxycytidine. The inhibitors of DNA excision repair, arabinosylcytosine, hydroxyurea or 3-aminobenzoamide increase the steady-state number of DNA breaks. Thus, they can be used to enhance even further the sensitivity of the nucleoid assay of repair.

Expression of a beta-globin gene is enhanced by remote SV40 DNA sequences

We have studied the transient expression of a cloned rabbit hemoglobin beta 1 gene after its introduction into HeLa cells. Two and one-half days after transfection using the calcium phosphate technique we extracted RNA from the entire cell population and analyzed it by the S1 nuclease hybridization assay. Transcripts were barely detectable when beta-globin gene-plasmid recombinants were used. However, 200 times more beta-globin gene transcripts were found when the beta-globin gene recombinants also contained SV40 DNA, and 90% of these transcripts (about 1000 per cell) had the same 5' end as authentic rabbit globin mRNA. In the latter case, abundant production of beta-globin protein was readily detected in a fraction of transfected cells by immunofluorescent staining. Enhancement of globin gene expression was dependent on SV40 sequences acting in cis, but independent of the viral origin of DNA replication. The enhancing activity was associated with the 72 bp repeated sequence element located at the beginning of the viral late gene region. Viral DNA fragments containing the transcriptional enhancer element could act in either orientation at many positions, including 1400 bp upstream or 3300 bp downstream from the transcription initiation site of the rabbit beta-globin gene. These studies define a class of DNA elements with a mode of action that has not been heretofore described. The activation of genes by specific enhancer elements seems to be a widespread mechanism that may be used for the regulation of gene expression.

Organization of highly purified calf thymus DNA. I. Cleavage into subunits and release of phosphopeptides

DNA (N-DNA) prepared under conditions eliminating the exposure of chromatin to cytoplasmic components exhibits some special properties not observed for DNA prepared by standard methods (S-DNA).N-DNA, having a sedimentation coefficient of 24.7 S and a firmly bound protein content of 0.7%, can be cleaved (in contrast to S-DNA) by treatment with chelating agents, into stable subunits having a mean molecular weight of about 500 000. This cleavage was shown to be an ordered process which involved no enzymatic or shear degradation. It was accompanied by the release of phosphopeptides. The analyses of these phosphopeptides revealed the presence of two main fractions. One contained phosphoserine and glycine (Mr about 1400), and the other contained phosphoserine, glycine, alanine, glutamic and aspartic acids (Mr about 900). The amount of released phosphopeptides could be correlated to the extent of cleavage.

Cellular and molecular aspects of immune system aging

We begin with a brief discussion of the importance and advantages of immune studies to the problem of aging. This is followed by a short over-view of immune system aging at the systemic level. The major portion of the article is a review of observation, both at the cellular and molecular level, of changes in aging immune cells, with sections on intercellular communication, membrane phenomena, cyclic nucleotides, and molecular genetic changes.

Extremely underwound chromosomal DNA in nucleoids of mouse sarcoma cells

The superhelical properties of chromosomal DNA from cells of a mouse sarcoma were investigated in neutral sucrose gradients containing ethidium bromide. Removal of negative supercoiling from the DNA of the sarcoma cells required a substantially higher dye concentration than was necessary in the case of DNA from cultured mouse fibroblasts. The calculated value of the mean superhelical density in malignant cells (sigma = -0.14) appears abnormally high compared with the value (sigma = -0.09) obtained for DNA of mouse fibroblasts. Chromosomal DNA from mouse sarcoma cells is therefore concluded to be highly deficient in helical turns.

Enrichment of selected active human gene sequences in the placental deoxyribonucleic acid fraction associated with tightly bound nonhistone chromosomal proteins

A DNA fraction which is highly enriched in active gene sequences and tightly associated with a subset of nonhistone chromosomal proteins has been isolated from human placenta. After extraction with 2 M NaCl, placental chromatin was separated into two distinct components by centrifugation. Of the total DNA, approximately 96% (DNA-S) is protein free, while the remaining 4% (DNA-P) is tightly complexed with nonhistone chromosomal proteins. Reassociation studies revealed that the DNA-P fraction was enriched 22-fold in actively transcribed human placental lactogen gene sequences, while the DNA-S fraction was correspondingly depleted 22-fold in these sequences. Approximately 45% of the sequences present in DNA-P (equivalent to 1.8% of the genome) were not present in the DNA-S fraction. Reassociation of nick-translated DNA-P to DNA from a partial digest of DNase I treated nuclei indicated that 27% of the DNA-P sequences were DNAase I sensitive, suggesting they may represent actively transcribed gene sequences. Analysis of the overall sequence organization of DNA-P showed that relative to unfractionated DNA and DNA-S, DNA-P was enriched in single-copy sequences, slightly enriched in the class of middle repetitive sequences from C0t 0.01 to 100 M.s, devoid of the more highly repetitive sequences (C0t less than or equal to 0.01). The distribution of total active placental genes between DNA-P and DNA-S was measured by hybridization with a complementary DNA probe transcribed from total polysomal poly(A+) messenger RNA. We found that 57% of this cDNA probe reassociated to DNA-P and 58% to DNA-S, while 95% reassociated to DNA-P mixed with DNA-S at the observed ratio of 4 to 96, suggesting that the DNA-P fraction contained a different population of active gene sequences than DNA-S. From these results we estimate that approximately 85% of the transcribed sequences appear to be distinctly distributed and equally proportioned between DNA-P and DNA-S, while approximately 15% of the transcribed sequences are common to both fractions. We suggest that the strong affinity of the tightly bound nonhistone chromosomal proteins for the DNA-P fraction indicates a likely role for these proteins in the regulation of gene expression.

Dependence of mammalian DNA synthesis on DNA supercoiling. III. Characterization of the inhibition of replicative and repair-type DNA synthesis by novobiocin and nalidixic acid

Novobiocin and nalidixic acid, inhibitors of the bacterial enzyme DNA gyrase, inhibit DNA, RNA and protein synthesis in several human and rodent cell lines. The sensitivity of DNA synthesis (both replicative and repair) to inhibition by novobiocin and nalidixic acid is greater than that of protein synthesis. Novobiocin inhibits RNA synthesis about half as effectively as it does DNA synthesis, whereas nalidixic acid inhibits both equally well. Replicative DNA synthesis, as measured by incorporation of [3H]thymidine, is blocked by novobiocin in a number of cell strains; the inhibition is reversible with respect to both DNA synthesis and cell killing, and continues for as long as 20--30 h if the cells are kept in novobiocin-containing growth medium. Both novobiocin and nalidixic acid inhibit repair DNA synthesis (measured by BND-cellulose chromatography) induced by ultraviolet light or N-methyl-N'-nitro-N-nitrosoguanidine (but not that induced by methyl methanesulfonate) at lower concentration (as low as 5 micrograms/ml) than those required to inhibit replicative DNA synthesis (50 micrograms/ml or greater). Neither novobiocin nor nalidixic acid alone induces DNA repair synthesis. Incubation of ultraviolet-irradiated cells with 10--100 micrograms/ml novobiocin results in little, if any, further reduction of colony-forming ability (beyond that caused by the ultraviolet irradiation). Novobiocin at sufficiently low concentrations (200 micrograms/ml) apparently generates a quiescent state (in terms of cellular DNA metabolism) from which recovery is possible. Under more drastic conditions of time in contact with cells and concentration, however, novobiocin itself induces mammalian cell killing.

A replication model for sister-chromatid exchange

A model for the production of sister-chromatid exchanges is presented, based on the idea that double-strand breaks are generated at junctions between a completely duplicated replicon cluster and a partially duplicated replicon cluster. Agents that induce absolute blocks to DNA fork displacement will cause this condition to persist longer than normal, whereas agents that inhibit initiation of whole clusters will rarely cause it at all. During the blunt-end repair of the double-strand breaks, sister-chromatod exchange would be initiated when daughter strands of a duplicated cluster recombine with the parental strands of the partially replicated cluster. When the latter finishes replication, sister-chromatid exchange would be completed.

Conformational transitions in closed circular DNA molecules. I. Topological and energetical considerations

A theory of conformational transitions in closed circular DNA as a function of topological linking number of the molecule (alpha) is elaborated taking into account topological and energetical considerations. The theory predicts a step-like dependence of a number of superhelical turns in DNA molecules (tau) on delta alpha. Thus, the number of superhelical turns tau = delta alpha for small values of delta alpha. For a large delta alpha (when conformational begin to occur) tau = delta alpha - sigma phi ij, where sigma phi ij is the total angle of conformational transitions for a given delta alpha. This prediction is in good agreement with published data on the dependence of the sedimentation coefficient of circular DNA molecules on their topological linking number. The results also allow to explain the disagreement between a number of titratable superhelical turns in circular DNA molecules and a number of supercoiles seen on electron micrographs for molecules with sufficiently large delta alpha.

Analysis of the most tightly bound proteins in eukaryotic DNA

DNA isolated by procedures generally considered to be most efficient for purifying DNA still contains detectable peptide components. The characteristics of this material and the stability of its linkage to DNA were investigated: DNA released from [35S]methionine-labelled cells by SDS in the presence of proteases contains a significant amount of 35S label which is not removed by additional treatment with proteases and phenol and which cosediments and cobands together with DNA on alkaline gradients. Furthermore, some peptide material which is copurified with native DNA and which remains complexed with DNA after alkali treatment can be labelled with 125I and analyzed on SDS-polyacrylamide-gels. The amino acid analysis of hydrolysates of purified DNA gives a rough estimate of the amount of the peptide material which is copurified with DNA. The results indicate that distinct proteins between 54 000 and 68 000 daltons in size are not removed from DNA by phenol, proteases, alkali or by any combination of these treatments. They can only be isolated by degradation of DNA. This extreme stability of the DNA-protein linkage indicates that these proteins are not merely contaminants which are difficult to eliminate but are rather covalently or otherwise bound (alkali-stable) to DNA. The size of these proteins and the stability of their linkage to DNA suggests that they are related to the class of non-histone proteins which are thought to be involved in chromatin structure e.g. by keeping DNA in a supercoiled state. Other possible functions are discussed.

Dependence of mammalian DNA replication on DNA supercoiling. I. Effects of ethidium bromide on DNA synthesis in permeable Chinese hamster ovary cells

Chinese hamster ovary cells labelled with [14C]thymidine were made permeable, incubated with various concentrations of the intercalating dye ethidium bromide, and centrifuged through neutral sucrose gradients. The gradient profiles of these cells were qualitatively similar to those obtained by centrifuging DNA from untreated, lysed permeable cells through gradients containing ethidium bromide. The sedimentation distance of DNA had a biphasic dependence on the concentration of ethidium bromide, suggesting that the dye altered the amount of DNA supercoiling in situ. The effect of ethidium bromide intercalation on incorporation of [3H]dTMP into acid-precipitable material in an in vitro DNA synthesis mixture was measured. The incorporation of [3H]dTMP was unaffected by less than 1 microgram/ml of ethidium bromide, enhanced up to two-fold by 1--10 microgram/ml, and inhibited by concentrations greater than 10 micrograms/ml. Alkaline sucrose gradient analysis revealed a higher percentage of small DNA fragments (6--20 S) in the cells treated with 2 micrograms/ml ethidium bromide than in control cells. These fragments attained parental size within the same time as the fragments in control cells. In cells treated with 2 micrograms/ml ethidium bromide, a significant fraction of newly synthesized DNA resulted from new starts, whereas in untreated cells practically none of the newly synthesized DNA resulted from new starts. These results suggest that relaxation of DNA supercoiled structures ahead of the replication fork generates spurious initiations of DNA synthesis and that in intact cells the rate of chain elongation is limited by supercoiled regions ahead of the growing point.

Dependence of mammalian DNA replication on DNA supercoiling. II. Effects of novobiocin on DNA synthesis in Chinese hamster ovary cells

Novobiocin, an inhibitor of gyrase-induced DNA supercoiling and DNA replication in prokaryotes, inhibited the incorporation of DNA precursors into DNA in both intact and permeable Chinese hamster ovary cells; much higher concentrations were required for permeable cells, in which no new replicons were initiated. Nucleoids were prepared from cells that were incubated for 60 min with 200 micrograms/ml novobiocin, made permeable, and incubated with 0--50 micrograms/ml ethidium bromide. Sedimentation of the nucleoids in neutral sucrose gradients suggested that the number of supercoils in the average nucleoid had been reduced by prior incubation with novobiocin. In intact cells, novobiocin is required inside the cell for continued inhibition of DNA synthesis, suggesting that it does not act directly on the DNA. Alkaline sucrose gradient profiles of DNA synthesized in the presence of novobiocin in intact cells indicated that the drug inhibited replicon initiation while having little if any effect on chain elongation. These data are consistent with the idea that an activity similar to the bacterial gyrase generates supercoils in mammalian DNA and produces the proper conformation for the initiation of DNA replication.

DNA repair defects and chromosome instability disorders

Xeroderma pigmentosum (XP), Fanconi anaemia (FA), ataxia telangiectasia (AT) and Bloom disease (BS) are four rare autosomal recessive disorders in which there is defective DNA repair and/or chromosome instability and proneness to malignancy. Between 80 and 90% of patients with XP have a defect, demonstrable at cell level, of excision of DNA lesions induced by ultraviolet rays, while the remainder have a cellular error of post-replication repair. XP cells are also deficient in repairing DNA damage caused by a variety of chemical mutagens. There are at least five different complementation groups of the first, or classical, type of XP (A to D, etc.) Apparently group C patients, as well as those with defective post-replication repair, do not show the progressive neurological illness found in a proportion of the other patients. AT is heterogeneous clinically and genetically. Clinically it presents with a progressive neurological illness, progressive telangiectases and a developmental disorder of the thymus. AT is characterized by sensitivity to X-rays and AT cells are unable to repair gamma-ray-induced damage to bases in the DNA. It appears that in many cases of the disorder a chromosomally marked cellular clone is found. In BS the main defect, which results in growth retardation, sun-induced lesions of the face and susceptibility to infection, appears to be a slow DNA chain maturation during DNA synthesis. An increase of sister chromatid exchanges is characteristically seen in the chromosomes of cultured BS cells. In FA, in which there is progressive pancytopenia with eventual bone marrow exhaustion and a tendency to haemorrhage and infection, the cellular defect seems to consist of faulty removal of repair of cross-links in the DNA. In this condition, as in BS and AT, various structural chromosome changes are detected in cultured cells. Patients with XP develop skin cancers in early life and often maligant melanomas. In the other three disorders, in which an immune deficiency is often present, leukaemia and related proliferative disorders are a frequent cause of death while other malignancies also occur. There is some evidence that points to an increased risk of malignancy in heterozygotes who carry the FA and AT genes.