A flow-stretching assay was employed to elucidate polycation/DNA interactions at the single-molecule level. Bacteriophage -DNA (48.5 kbp) was attached at one end to a PEGmodified glass surface and stretched by buffer flow. The stretched DNA had an approximate length of 12¯m. Upon injection of polylysine homopolymer, the DNA folded into a globule structure within a few seconds. Injection of a polylysine graft copolymer having hydrophilic dextran side chains also induced collapse of the stretched DNA. In comparison to these polymers, a copolymer with higher graft content did not cause DNA collapse but rather caused 25% shrinkage of the extended DNA. These results were compared with those observed with unstretched DNA.Long DNA molecules can undergo discrete coil to globule transitions. Polycations like polylysine are effective inducers of the transition.1 Upon complex formation with polycations, the DNA molecules collapse into compact globule conformations. The complexes subsequently aggregate and precipitate from the solution. We have been interested in soluble complex formation observed with cationic graft copolymers containing poly(Llysine) and dextran, PLL-g-Dex ( Figure 1); PLL-g-Dex has a polycation backbone and hydrophilic side chains.2 The copolymer with a high degree of grafting forms totally soluble DNA complexes without inducing collapse. 3 We have shown that PLL-g-Dex increases stability of DNA hybrids, 2 accelerates DNA hybridization, 4 and activates DNA strand-exchange reactions.5 However, interaction of the copolymer with DNA is not thoroughly understood and the influence of the copolymer grafting degree on conformation of DNA has not been assessed. Here, we employed a DNA flow-stretching assay to further evaluate DNA/polycation interactions. The DNA flow-stretching assay has high spatiotemporal resolution and has been used to study DNA/protein interactions. 6 The results of the flowstretching assay were compared to that from assays with DNA and polycations in solution.PLL-g-Dex copolymers (M n of PLL: 4.0 © 10 4 , M n of Dex: 8.4 © 10 3 , Dex grafts: 38, 61, and 91 wt %) were prepared as previously reported.2 The nomenclature used for the copolymers reflects the degree of grafting: Polymers with grafts of 38, 61, and 91 wt % are referred to as 40K38D, 40K61D, and 40K91D, respectively. First, we observed the effect of these copolymers on conformation of bacteriophage -DNA (48.5 kbp) in aqueous buffer solution using a method described by Yoshikawa et al. 7 We observed images of cationic polymers in the same mixtures by employing a second fluorescent label. As shown in Figure 2, 11 DNA was visualized with SYBR Gold staining (488 nm excitation) and polymers were imaged (561 excitation) by TRITC labeling. The -DNA alone shows amorphous fluorescence images with 35¯m long axes on average. The DNA images wobbled due to Brownian motion, indicating an extended coil conformation of the DNA. When polylysine homopolymer (40K) was added to the DNA solution, bright DNA clots were observed, indicating that the 40K ...

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