Direct Observation of Strand Passage by DNA-Topoisomerase and Its Limited Processivity

Type-II DNA topoisomerases resolve DNA entanglements such as supercoils, knots and catenanes by passing one segment of DNA duplex through a transient enzyme-bridged double-stranded break in another segment. The ATP-dependent passage reaction has previously been demonstrated at the single-molecule level, showing apparent processivity at saturating ATP. Here we directly observed the strand passage by human topoisomerase IIα, after winding a pair of fluorescently stained DNA molecules with optical tweezers for 30 turns into an X-shaped braid. On average 0.51±0.33 µm (11±6 turns) of a braid was unlinked in a burst of reactions taking 8±4 s, the unlinked length being essentially independent of the enzyme concentration between 0.25–37 pM. The time elapsed before the start of processive unlinking decreased with the enzyme concentration, being ∼100 s at 3.7 pM. These results are consistent with a scenario where the enzyme binds to one DNA for a period of ∼10 s, waiting for multiple diffusional encounters with the other DNA to transport it across the break ∼10 times, and then dissociates from the binding site without waiting for the exhaustion of transportable DNA segments

Transdermal Delivery of Small Interfering RNA with Elastic Cationic Liposomes in Mice

We developed elastic cationic liposomal vectors for transdermal siRNA delivery. These liposomes were prepared with 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) as a cationic lipid and sodium cholate (NaChol) or Tween 80 as an edge activator. When NaChol or Tween 80 was included at 5, 10, and 15% (w/w) into DOTAP liposomal formulations (C5-, C10-, and C15-liposomes and T5-, T10-, and T15-liposomes), C15- and T10-liposomes showed 2.4- and 2.7-fold-higher elasticities than DOTAP liposome, respectively. Although the sizes of all elastic liposomes prepared in this study were about 80–90 nm, the sizes of C5-, C10- and C15-liposome/siRNA complexes (lipoplexes) were about 1,700–1,800 nm, and those of T5-, T10-, and T15-lipoplexes were about 550–780 nm. Their elastic lipoplexes showed strong gene suppression by siRNA without cytotoxicity when transfected into human cervical carcinoma SiHa cells. Following skin application of the fluorescence-labeled lipoplexes in mice, among the elastic lipoplexes, C15- and T5-lipoplexes showed effective penetration of siRNA into skin, compared with DOTAP lipoplex and free siRNA solution. These data suggest that elastic cationic liposomes containing an appropriate amount of NaChol or Tween 80 as an edge activator could deliver siRNA transdermally

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a molecular knot with optical tweezer