Vertebrate telomere repeat DNAs favor external loop propeller quadruplex structures in the presence of high concentrations of potassium

The circular dichroism, CD, spectra of the telomere repeats of vertebrates, d(TTAGGG), indicate that parallel type quadruplex structures or disordered single-stranded structures are formed in low salt. Anti-parallel quadruplex structures are favored in the presence of high concentrations, 140 mM, of sodium. External loop, also known as propeller, parallel type structures are favored in the presence of high concentrations, 100 mM, of potassium in the presence of either 5 or 140 mM sodium. The cation dependence of the CD spectra of the vertebrate telomere repeat DNAs is distinctly different from that of the telomere repeats of Tetrahymena and Oxytricha as well as that of the thrombin binding aptamer. These results indicate that the external loop structures may be present in vertebrate telomeres under the conditions of high potassium and low sodium concentration found in nuclei

Identification of inhibitors of Plasmodium falciparum phosphoethanolamine methyltransferase using an enzyme-coupled transmethylation assay

<p>Abstract</p> <p>Background</p> <p>The phosphoethanolamine methyltransferase, PfPMT, of the human malaria parasite <it>Plasmodium falciparum</it>, a member of a newly identified family of phosphoethanolamine methyltransferases (PMT) found solely in some protozoa, nematodes, frogs, and plants, is involved in the synthesis of the major membrane phospholipid, phosphatidylcholine. PMT enzymes catalyze a three-step S-adenosylmethionine-dependent methylation of the nitrogen atom of phosphoethanolamine to form phosphocholine. In <it>P. falciparum</it>, this activity is a limiting step in the pathway of synthesis of phosphatidylcholine from serine and plays an important role in the development, replication and survival of the parasite within human red blood cells.</p> <p>Results</p> <p>We have employed an enzyme-coupled methylation assay to screen for potential inhibitors of PfPMT. In addition to hexadecyltrimethylammonium, previously known to inhibit PfPMT, two compounds dodecyltrimethylammonium and amodiaquine were also found to inhibit PfPMT activity <it>in vitro</it>. Interestingly, PfPMT activity was not inhibited by the amodiaquine analog, chloroquine, or other aminoquinolines, amino alcohols, or histamine methyltransferase inhibitors. Using yeast as a surrogate system we found that unlike wild-type cells, yeast mutants that rely on PfPMT for survival were sensitive to amodiaquine, and their phosphatidylcholine biosynthesis was inhibited by this compound. Furthermore NMR titration studies to characterize the interaction between amoidaquine and PfPMT demonstrated a specific and concentration dependent binding of the compound to the enzyme.</p> <p>Conclusion</p> <p>The identification of amodiaquine as an inhibitor of PfPMT <it>in vitro </it>and in yeast, and the biophysical evidence for the specific interaction of the compound with the enzyme will set the stage for the development of analogs of this drug that specifically inhibit this enzyme and possibly other PMTs.</p