Our observation reveals the effects of divalent and trivalent cations on the higher-order structure of giant DNA (T4 DNA 166 kbp) by fluorescence microscopy. It was found that divalent cations, Mg(2+) and Ca(2+), inhibit DNA compaction induced by a trivalent cation, spermidine (SPD(3+)).On the other hand, in the absence of SPD(3+), divalent cations cause the shrinkage of DNA.As the control experiment, we have confirmed the minimum effect of monovalent cation, Na(+) on the DNA higher-order structure. We interpret the competition between 2+ and 3+ cations in terms of the change in the translational entropy of the counterions. For the compaction with SPD(3+), we consider the increase in translational entropy due to the ion-exchange of the intrinsic monovalent cations condensing on a highly charged polyelectrolyte, double-stranded DNA, by the 3+ cations. In contrast, the presence of 2+ cation decreases the gain of entropy contribution by the ion-exchange between monovalent and 3+ ions. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

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The repetitive TG-rich DNA sequence at the chromosome end, telomere, protects cells from the end-replication problem in eukaryotic cells. S. cerevisiae telomerase (Est2) is the reverse transcriptase responsible for extending telomeres in yeast. Pif1 helicase has been implicated in regulating the telomerase activity. We used single-molecule experiments to investigate how Pif1 helicases regulate telomerase activity. We found that Est2 telomerase stayed bound to telomere end after extension, but Pif1 helicases remove telomerase from the telomere. In the presence of Pif1 helicases, multiple runs of the telomerase-mediated telomere lengthening were observed. This suggests a model that Pif1 helicases remove telomerase from the telomere ends, allowing telomerase recycling.