Turnover-synthesis of chloroplast DNA in developing chloroplasts.

The mechanism for the turnover-synthesis of chloroplast DNA in the absence of net synthesis during the chloroplast maturation in Euglena gracilis was determined. DNA synthesis was measured by incorporation of32Pi into chloroplast DNA. The density label, 15N, was incorporated to examine the mechanism of turnover-synthesis. The newly synthesized segments represent a replacement of segments in the DNA containing 1.5 X 10(3) to 6.1 X 10(3) nucleotides. Twenty-three fragments of chloroplast DNA, generated by digestion with the restriction endonuclease EcoRI, became labeled with 32Pi. Turnover-synthesis, therefore, replaces segments throughout the molecule of chloroplast DNA.     

Some effects of tentoxin on mature and developing chloroplasts

Tentoxin affected chloroplasts of all stages of development; it caused chlorosis of expanded and developing bean and lettuce leaves when introduced through the stem or roots and inhibited greening of etiolated lettuce seedlings. Cotyledons from 6-day-old lettuce seedlings grown in 10ug/ml tentoxin bad 50–68% of the monogalactosyldiglyceride and digalactosyldiglyceride content of healthy tissue whereas the sulfolipid, phospatidyl-glycerol, -ethanolamine, -choline and -inosrtol levels were unchanged. Toxin-treated tissue also showed selective increases in C-16 and C-18 saturated fatty acids.Research cooperative with the College of Agricultural and Life Sciences, University of Wisconsin, Madison, and the Agricultural Research Service, U.S. Department of Agriculture.Mention of companies or commercial products does not imply recommendation or endorsement by the U.S. Department of Agriculture over others not mentioned.     

Different effects of eubacterial and eukaryotic DNA topoisomerase II inhibitors on chloroplasts ofEuglena gracilis

Inhibitors of eubacterial and eukaryotic DNA topoisomerases type II exhibited different effects on chloroplasts of the flagellateEuglena gracilis. Antibacterial agents (cinoxacin, nalidixic and oxolinic acids, ciprofloxacin, enoxacin, norfloxacin and ofloxacin) from the group of quinolones and coumarins (coumermycin A₁, clorobiocin and novobiocin) — all inhibitors of prokaryotic DNA topoisomerase II — were very potent eliminators of chloroplasts fromE. gracilis. In contrast, antitumor drugs (adriamycin, etoposide, teniposide and mitoxantrone) — antagonists of the eukaryotic counterpart — did not affect these semiautonomous photosynthetic organelles. These findings point out again the close evolutionary relationships between eubacteria and chloroplasts and are in agreement with the hypothesis of an endosymbiotic origin of chloroplasts.     

Changes in distribution of nucleoids in developing and dividing chloroplasts and etioplasts ofArena sativa

Summary Nucleoid distribution in chloroplasts and etioplasts at the different developmental stages was examined with the first leaves ofAvena sativa by using a DNA-specific fluorescent probe, 46-diamidino-2-phenylindole (DAPI). In light-grown first leaves, three types of plastid nucleoid distribution were recognized. 1. Peripheral distribution in undeveloped chloroplasts which contain only a few thylakoids in the middle region of the leaf sheath. 2. Ring-like arrangement along the rim of developing and dividing young chloroplasts, of which grana were composed of four to eight layers of thylakoids, at the base of the leaf blade. The plane of the nucleoids' ring is in parallel with the face of the thylakoids. 3. Scattered distribution of 10 to 20 discrete spherular nucleoids in the stroma of fully developed chloroplasts, of which grana were composed of up to 20 thylakoids, in the regions of the middle and the tip of the leaf blade. In dark-grown first leaves two types were recognized. 1. Peripheral distribution in developing and dividing young etioplasts in the leaf sheath and the base of the leaf blade. 2. Scattered distribution of 10 or more discrete spherular nucleoids in fully developed etioplasts, containing extended prothylakoids, in the regions of the middle and the tip of the leaf blade. Ring-like arrangement of nucleoids was not observed in any etioplasts. The results indicates that spatial arrangement of plastid nucleoids dynamically changes in close relationship with the development of the inner membrane systems of plastids.     

DNA topoisomerase II, genotoxicity, and cancer

Type II topoisomerases are ubiquitous enzymes that play essential roles in a number of fundamental DNA processes. They regulate DNA under- and overwinding, and resolve knots and tangles in the genetic material by passing an intact double helix through a transient double-stranded break that they generate in a separate segment of DNA. Because type II topoisomerases generate DNA strand breaks as a requisite intermediate in their catalytic cycle, they have the potential to fragment the genome every time they function. Thus, while these enzymes are essential to the survival of proliferating cells, they also have significant genotoxic effects. This latter aspect of type II topoisomerase has been exploited for the development of several classes of anticancer drugs that are widely employed for the clinical treatment of human malignancies. However, considerable evidence indicates that these enzymes also trigger specific leukemic chromosomal translocations. In light of the impact, both positive and negative, of type II topoisomerases on human cells, it is important to understand how these enzymes function and how their actions can destabilize the genome. This article discusses both aspects of human type II topoisomerases.     

Isolation of chloroplasts and chloroplast DNA from pea leaves]

A method of isolating DNA from pea chloroplasts (ch-DNA) in CsCl density gradient is described. DNA preparations are free of 5-methylcytosine and have a melting temperature of 86.5 degrees. Denatured DNA molecules completely reassociate for 3 hours at 60 degrees C. It is concluded that the preparations obtained are pure ch-DNA.     

Cleavage of DNA by mammalian DNA topoisomerase II

Using the P4 unknotting assay, DNA topoisomerase II has been purified from several mammalian cells. Similar to prokaryotic DNA gyrase, mammalian DNA topoisomerase II can cleave double-stranded DNA and be trapped as a covalent protein-DNA complex. This cleavage reaction requires protein denaturant treatment of the topoisomerase II-DNA complex and is reversible with respect to salt and temperature. The product after reversal of the cleavage reaction remains supertwisted, suggesting that the two ends of the putatively broken DNA are held tightly by the topoisomerase. Alternatively, the enzyme-DNA interaction is noncovalent, and the covalent linking of topoisomerase to DNA is induced by the protein denaturant. Detailed characterization of the cleavage products has revealed that topoisomerase II cuts DNA with a four-base stagger and is covalently linked to the protruding 5'-phosphoryl ends of each broken DNA strand. Calf thymus DNA topoisomerase II cuts SV40 DNA at multiple and specific sites. However, no sequence homology has been found among the cleavage sites as determined by direct nucleotide-sequencing studies.     

Yeast DNA topoisomerase II. An ATP-dependent type II topoisomerase that catalyzes the catenation, decatenation, unknotting, and relaxation of double-stranded DNA rings

An activity from the yeast Saccharomyces cerevisiae, initially noted for its catalysis of aggregation of covalently closed double-stranded DNA rings in the presence of ATP, has been identified as a type II DNA topoisomerase and is designated yeast DNA topoisomerase II. The formation of the DNA aggregate, which has been shown to be a network of DNA rings that are topologically interlocked, requires the presence of a yeast DNA-binding protein in addition to the topoisomerase. In the absence of the binding protein, yeast DNA topoisomerase II catalyzes decatenation and unknotting of duplex DNA rings and the relaxation of negatively or positively supercoiled DNA. All reactions are ATP-dependent and require Mg(II). Similar to other eukaryotic and phage T4-type II DNA topoisomerases, the yeast enzyme does not catalyze DNA supercoiling under the assay conditions employed. The activity is not sensitive to the gyrase inhibitor nalidixic acid, oxolinic acid, or novobiocin. Coumermycin inhibits the activity, however, at a concentration as low as 5 microgram/ml.     

Defective DNA topoisomerase II in ataxia-telangiectasia cells

A number of characteristics in the human genetic disorder ataxia-telangiectasia are compatible with an alteration to chromatin structure or the recognition of that structure by an enzyme or DNA binding protein. We describe here reduce activity of DNA topoisomerase type II in a number of Epstein Barr Virus-transformed ataxia-telangiectasia lymphoblastoid cell lines. Enzyme activity was reduced 10-fold or greater in 4 out of 5 cell lines compared to controls. In the remaining cell line approximately a 2-3 fold reduction was evident in partially purified extracts. DNA topoisomerase type I activity was found to be the same as controls in all the cell lines. Northern blot analysis revealed that the same level of DNA topoisomerase II mRNA was expressed in ataxia-telangiectasia and control cell lines. The size and amount of the enzyme did not differ appreciably from that observed in control cells. The reduced activity of DNA topoisomerase II in ataxia-telangiectasis cells might be explained by amino acid substitutions, small deletions in DNA or by a defect in post-translational modification in these cells.     

Altered DNA topoisomerase II in multidrug resistance

The characteristic feature of multidrug resistance (MDR) associated with drugs that interact with DNA topoisomerase II (topo II) is alterations in topo II activity or amount (at-MDR). We have characterized the at-MDR phenotype in human leukemic CEM cells selected for resistance to the topo II inhibitor, VM-26. Compared to drug-sensitive cells, the key findings are that at-MDR cells exhibit (i) decreased topo II activity; (ii) decreased drug sensitivity, activity and amount of nuclear matrix topo II; (iii) increased ATP requirement of topo II; (iv) a single base mutation in topo II resulting in a change of Arg to Gln at position 449, at the start of the motif B/nucleotide binding site; and (v) decreased topo II phosphorylation, suggesting decreased kinase or increased phosphatase activities. Recent results using single-stranded conformational polymorphism analysis reveals the presence of a mutation in the motif B/nucleotide binding site of the topo II gene in CEM at-MDR cells and in another leukemic cell line selected for resistance to m-AMSA. Finally, we have observed marked changes in the nuclear distribution of topo II in cells treated with anti-topo II drugs and have also found these changes to be attenuated in drug-resistant cells. We postulate that traditional inhibitors of topo II alter the equilibrium of the strand-passing reaction such that the number of enzyme-DNA covalent complexes increases. We further suggest that when the enzyme is bound to DNA it is protected from proteolysis, thus allowing more topo II molecules to be detected. We propose that MDR associated with alterations in topo II may have clinical consequences, and our current efforts involve exploiting these biochemical and molecular observations in the development of probes that may be useful to identify such drug resistant cells in the tumors of patients.     

Amiloride intercalates into DNA and inhibits DNA topoisomerase II

Amiloride is capable of inhibiting DNA synthesis in mammalian cells in culture. Recent evidence indicates that the enzyme, DNA topoisomerase II, is probably required for DNA synthesis to occur in situ. In experiments to determine the mechanism of inhibition of DNA synthesis by amiloride, we observed that amiloride inhibited both the catalytic activity of purified DNA topoisomerase II in vitro and DNA topoisomerase II-dependent cell functions in vivo. Many compounds capable of inhibiting DNA topoisomerase II are DNA intercalators. Thus, we performed studies to determine if and how amiloride bound to DNA. Results indicated that amiloride 1) shifted the thermal denaturation profile of DNA, 2) increased the viscosity of linear DNA, and 3) unwound circular DNA, all behavior consistent with a DNA intercalation mechanism. Furthermore, quantitative and qualitative measurements of amiloride fluorescence indicated that amiloride (a) bound reversibly to purified DNA under conditions of physiologic ionic strength, and (b) bound to purified nuclei in a highly cooperative manner. Lastly, amiloride did not promote the cleavage of DNA in the presence of DNA topoisomerase II, indicating that the mechanism by which amiloride inhibited DNA topoisomerase II was not through the stabilization of a "cleavable complex" formed between DNA topoisomerase II, DNA, and amiloride. The ability of amiloride to intercalate with DNA and inhibit topoisomerase II is consistent with the proposed planar, hydrogen-bonded, tricyclic nature of amiloride's most stable conformation. Thus, DNA and DNA topoisomerase II must be considered as new cellular targets of amiloride action.     

Leucine zipper in human DNA topoisomerase II

Examination of the amino acid sequence of human DNA topoisomerase II revealed the presence of a leucine zipper, a novel motif found in several proteins localized to the cell nucleus. The presence of this motif in this unique protein may explain some of the normal functions of topoisomerase II as well as the disruption of those functions by antineoplastic drugs.     

Synthesis, cytotoxicity, and DNA topoisomerase II inhibitory activity of benzofuroquinolinediones

Benzofuroquinolinediones (7c and 7d) were synthesized by base-catalyzed condensation of dichloroquinolinediones with phenolic derivatives. Their dialkylaminoalkoxy derivatives (8i-8p) were prepared by reaction with various dialkylaminoalkyl chlorides. The cytotoxicity of the synthesized compounds was evaluated against eight types of human cancer cell lines, and their topoisomerase II inhibition was assessed. In general, the cytotoxicity of benzofuroquinolinediones (8i-8p) was similar or superior to that of doxorubicin and showed more potent inhibitory activity than naphthofurandiones (8a-8h). Also, most of the compounds exhibited excellent topoisomerase II inhibitory activity at a concentration of 5 microM and two compounds, 8d and 8i, showed IC50 values of 1.19 and 0.68 microM, respectively, and were much more potent than etoposide (IC50=78.4 microM), but similar to doxorubicin (IC50=2.67 microM). However their inhibitory activity on topoisomerase I was lower, and 8d and 8i showed IC50 values of 42.0 and 64.3 microM, respectively.     

Interleukin-2 induces the activities of DNA topoisomerase I and DNA topoisomerase II in HuT 78 cells

The induction by interleukin-2 of DNA topoisomerase I and DNA topoisomerase II activities in the human T cell line HuT 78 was investigated. HuT 78 cells were treated with 1000 U of interleukin-2/ml, and extracts of the HuT 78 nuclei were prepared over a 24 h period. The extracts were assayed quantitatively for the activities of DNA topoisomerase I and DNA topoisomerase II. Three concomitant, transient increases of 3- to 11-fold in the specific activities of both DNA topoisomerase I and DNA topoisomerase II were observed following treatment with IL-2 at 0.5, 4, and 10 h after treatment with interleukin-2. The specific activities of both enzymes returned to base-line values after each of these transient increases. These results reveal that the activities of DNA topoisomerase I and DNA topoisomerase II are highly regulated in HuT 78 cells upon treatment with IL-2.     

Developmental regulation of Drosophila DNA topoisomerase II

Affinity-purified polyclonal antibodies were used to quantitate steady-state levels of DNA topoisomerase II (topo II) throughout Drosophila development. Although wide fluctuations were recorded at different stages, these fluctuations were paralleled by changes in levels of the nuclear lamin, a nuclear structural protein used as an internal standard. The exception to this was adult males where lamin levels were significantly elevated relative to topo II. Northern blot analyses of topo II and lamin mRNA, performed in conjunction with immunoblot analyses of protein revealed fluctuations in levels of the two different messages that paralleled changes in each other and in their respective translation products. Biochemical and immunochemical analyses were complemented by indirect immunofluorescence and immunoperoxidase experiments performed in situ. topo II was found distributed throughout nuclei in most but not all cell types examined. These results for Drosophila topo II are apparently at odds with those obtained by others working in vertebrate systems (see, for example, Heck, M. M. S. and W. C. Earnshaw. 1986. J. Cell Biol. 103:2569-2581; Heck, M. M. S., W. N. Hittelman, and W. C. Earnshaw. 1988. Proc. Natl. Acad. Sci. USA. 85:1086-1090) and suggest that in Drosophila, topo II may not be a useful marker for the proliferative state.