Ecological aspects of a silicified bivalve fauna from the Silurian of Gotland

The silicified Wenlockian (Silurian) bivalve fauna from MÖllbos, Gotland, is part of a life assemblage. The vast number of shells show unusual phenomena, e.g. shell repair, pearl and tumour formation, etc. A number of shells contain epibionts and bored, round holes. Presumptive predators of the bivalve community are discussed. Size-frequency distribution of the two most abundant species possibly reflects age classes. The fauna, comprising eleven species, is dominated by deposit-feeders (90 %). They exhibit niche diversification, including at least three different feeding levels within the sediment.     

The effect of various inhibitors of DNA synthesis on the repair of DNA photoproducts

The effect on DNA repair of several inhibitors of DNA synthesis has been investigated in CHO cells. Three assays were employed following ultraviolet irradiation of G₁ cells: unscheduled DNA synthesis, removal of antibody binding sites and alkaline elution. Cytosine arabinoside and aphidicolin were found to reduce unscheduled DNA synthesis in a dose-dependent manner without affecting the removal of antibody-binding sites. Strand rejoining was also inhibited. These results are consistent with the hypothesis that inhibition is due to premature chain termination during repair synthesis some time after excision of the lesion. Conversely, inhibition of unscheduled DNA synthesis by novobiocin is paralleled by inhibition of excision of the lesion. However, no inhibition of incision was apparent. Since nalidixic acid, an inhibitor of topoisomerase II, did not inhibit excision, it is unlikely that the primary site of action of novobiocin is this topoisomerase. The possibility that a second topoisomerase and/or a polymerase are affected is discussed in the light of previously published data.     

Analysis of the distribution of DNA repair patches in the DNA-nuclear matrix complex from human cells

The distribution of ultraviolet-induced repair patches along DNA loops attached to the nuclear matrix, was investigated by digestion with DNA-degrading enzymes and neutral sucrose gradient centrifugation. When DNA was gradually removed by DNAase 1, pulse label incorporated by ultraviolet-irradiated cells during 10 min in the presence of hydroxyurea or hydroxyurea/arabinosylcytosine showed similar degradation kinetics as prelabelled DNA. No preferential association of pulse label with the nuclear matrix was observed, neither within 30 min nor 13 h after irradiation. When the pulse label was incorporated by replicative synthesis under the same conditions, a preferential association of newly-synthesized DNA with the nuclear matrix was observed. Single-strand specific digestion with nuclease S₁ of nuclear lysates from ultraviolet-irradiated cells, pulse labelled in the presence of hydroxyurea/arabinosylcytosine, caused a release of about 70% of the prelabelled DNA and 90% of the pulse-labelled DNA from the rapidly sedimenting material in sucrose gradients. The results suggest no specific involvement of the nuclear matrix in repair synthesis, a random distribution of repair patches along the DNA loops, and simultaneously multiple incision events per DNA loop.     

Enhancement of the repair of carcinogen-induced DNA damage in the hamster pancreas by dietary selenium

We measured single strand breaks (SSB) in pancreas DNA produced by N-nitrosobis(2-oxopropyl)amine (BOP) in hamsters fed purified diets containing added sodium selenite (Se) at 0.0, 0.1 and 5.0 ppm. There were fewer SSB in those given the 5.0 ppm Se diet throughout the experiment. One hour after dosing with BOP (20 mg/kg), there were 2.26 ± 0.47, 2.83 ± 0.43 and 1.74 ± 0.43 SSB per 10⁸ daltons (mean ± S.E.M.) respectively in the three groups. The SSB were repaired faster in the 5.0 ppm Se-fed group. The approximate half-lives of the SSB were 33, 30 and 8 days, respectively. In the hamsters fed 5.0 ppm Se there was a small, statistically significant increase in pancreatic DNA synthesis. Autoradiographic analysis indicated that this was repair synthesis. In a second experiment, hamsters were fed one of the three diets prior to and for 2 days after administration of a single dose of BOP (20 mg/kg). They were then fed the 5.0 ppm Se diet for 5 days. The number of SSB was compared with those in hamsters fed their original diet for 7 days after BOP dosing. There was a statistically significant difference in the number of SSB in the hamsters fed 0.1 ppm Se before and for 2 days after BOP. In these hamsters there were 1.21 ± 0.24 SSB per 10⁸ daltons compared with 3.19 ± 0.4 (mean ± S.E.M.). These results suggest high levels of dietary Se stimulate the repair of carcinogen-induced DNA damage.     

聚乙二醇处理大豆种子子叶中几种酶活性和可溶性蛋白含量的变化

聚乙二醇处理的大豆种子的异柠檬酸裂解酶、苹果酸脱氢酶、过氧化氢酶、超氧物歧化酶、酸性磷酸酶、碱性磷酸酶的活性明显高于受低温吸胀冷害的种子子叶的活性,相关的酶活性协同地增长,而蛋白质的含量没有明显的变化。这些酶活性的提高可能是渗透调控处理对细胞膜系统修补的结果。     

Site-directed mutagenesis of the T4 endonuclease V gene: mutations which enhance enzyme specific activity at low salt concentrations

Previous structure/function analyses of the DNA repair enzyme, T4 endonuclease V, have suggested that the extreme carboxyl portion of the enzyme is associated with pyrimidine dimer-specific binding (Recinos and Lloyd, and Stump and Lloyd, Biochemistry 27:1832-1838 and 1839-1843, 1988, respectively). Within the final 11 amino acids there are 5 aromatic, 2 basic, and no acidic residues and it has been proposed that these residues stack with and electrostatically interact with the kinked DNA at the site of a pyrimidine dimer. The role of the tyrosine residue at position 129 has been investigated by oligonucleotide site-directed mutagenesis in which the codon for Tyr-129 has been altered to reflect conservative changes of Trp and Phe and more dramatic changes of Ser, a stop codon, deletion of the codon or introduction of a frameshift. Both changes to the aromatic amino acids resulted in proteins which accumulated well in E. coli and not only significantly enhanced the UV survival of repair-deficient cells but also complemented a defective denV gene within UV-irradiated T4 phage. Partially purified preparations of the Tyr-129----Trp and Tyr-129----Phe mutants were assayed for their ability to processively incise UV-irradiated plasmid DNA (a nicking reaction carried out at low 25 mM salt concentrations). The mutant enzymes Tyr-129----Phe and Tyr-129----Trp displayed a 1000% and 500% enhanced specific nicking activity, respectively. These reactions were also shown to be completely processive. Assays performed at higher (100 mM) salt concentrations reduced the specific activities of the mutant enzymes approximately to that of wild type for the Tyr-129----Phe mutant and to 20% that of wild type for the Tyr-129----Trp mutant.     

Molecular and functional analysis of the ruv region of Escherichia coli K-12 reveals three genes involved in DNA repair and recombination

Summary Recombinant plasmids carrying ruvA, ruvB, or both were constructed and used to investigate the genetic defects in a collection of UV-sensitive ruv mutants. The results revealed that efficient survival of UV-irradiated cells depends on both ruvA and ruvB, and on a third gene, ruvC, located upstream of the ruvAB operon. Southern blotting analysis was used to locate insertions in ruv and to examine putative deletion mutants. Two Tn10 insertions were located to the region encoding ruvA. Since these insertions caused a deficiency in the activities of both ruvA and ruvB, we concluded that they must exert a polar effect on ruvB. Two putative ruv deletion mutants were shown to be the result of deletion-inversion events mediated during imprecise excision of Tn10. The relevant inversion breakpoints in these mutants were located to ruvA and ruvC.     

A high level of hybrid dysgenesis in Drosophila: High thermosensitivity, dependence on DNA repair, and incomplete cytotype regulation

Summary An unusually high level of P-M hybrid dysgenesis in Drosophila melanogaster is characteristic of hybrid offspring originating from both, A (M × ) and B (P × M) crosses of a subline of the Harwich P strain, termed H ^s . The novel properties induced by mobility of P elements carried by H ^s paternal chromosomes include: very high (over 95%) gonadal dysgenesis (GD) in both sexes at the low restrictive temperature of 21°C, and highly premature sterility when males are reared at 18°C and aged at 21°C. Although all three major chromosomes of the H ^s subline contributed to this atypical pattern of gonadal dysgenesis, chromosome 3 had the largest effect. Gonadal dysgenesis showed a temperature- and sex-dependent repression pattern by the defective P elements of Muller-5 Birmingham chromosomes; at 21°C there was virtually no repression of male sterility, but most effective repression of GD in females. At 29°C repression was effective in males, but declined in females. The high thermosensitive sterility, low fecundity, and premature aging of the male germ line were greatly exacerbated when males derived from either A or B crosses were deficient either in excision repair (mei-9 mutation) or in post-replication repair (mei-41 mutation). These findings demonstrate that both DNA repair pathways are essential for the repair of lesions induced by P element transposition and support the hypothesis that P element-induced chromosome breaks are responsible for the virtual abolition of the germ line. The relatively high premature sterility of cross B DNA repair-deficient males, reared at 18°C and aged at 21°C, indicates that there is incomplete cytotype regulation in H ^s subline hybrids.     

DNA sequence analysis of spontaneous histidine mutations in a polA1 strain of Escherichia coli K12 suggests a specific role of the GTGG sequence

Summary Spontaneously arising histidine mutations in an Escherichia coli K12 strain deficient for DNA polymerase I were analysed at the DNA sequence level. We screened approximately 150000 colonies and isolated 106 histidine auxotrophs. Of these, 98 were unstable hisC mutations; 12 representative mutants analysed were shown to have arisen by the excision of a single quadruplet repeat in the sequence 5-GCTGGCTGGCTGGCTG-3. Of the eight mutations at other sites, three hisA deletions and one hisD deletion occurred as a consequence of misalignment of tandemly repeated pentamers (hisD) or decamers (hisA). A single hisA point mutation was found to be a missense mutation. Two extended deletions, covering the his operon were not analysed. We could not identify the hisC deletion by sequencing. We conclude that polA1 is a strong imitator that induces mutations mostly of the minus frameshift and deletion type by a Streisinger-type of mispairing in repetitive DNA sequences. Finally, the possible role of a 5-GTGG-3 sequence and its inverted or direct complements, which are found in the vicinity of all the deletions and frameshifts, is discussed.