Controlling the folding/unfolding transition of the DNA± histone H1 complex by direct optical manipulation Yuko Yoshikawa a, * , Shin-ichirou M. Nomura b , Toshio Kanbe c , Kenichi Yoshikawa b a Department of Food and Nutrition, Nagoya Bunri College, Nagoya 451-0077, Japan b Department of Physics and CREST, Kyoto University, Kyoto 606-8502, Japan c Laboratory of Medical Mycology, Research Institute for Disease Mechanism and Control, Nagoya University School of Medicine, Nagoya 464-0064, Japan Received 24 July 2000; in ®nal form 16 August 2000 Abstract Single giant duplex DNA (T4DNA, 166 kbp) complexed with a polycationic protein, histone H1, is trapped by a continuous-wave infrared laser, without chemical modi®cation of the DNA such as attachment to a micrometer-sized bead. When the DNA±H1 complex is transported from a low-salt solution (0.2 M NaCl) to a high-salt solution (2 M NaCl), its conformation changes from a compact to an elongated structure. With the reverse procedure, i.e., with the transportation from the high-salt to the low-salt environment, the elongated chain shrinks into a compact state. These results aord the direct evidence that histone H1 induces a salt-dependent transition in the higher-order structure of giant DNA molecules. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Recently, there has been a marked development in the methodologies to observe and manipulate single biopolymers [1±8]. For example, laser trap- ping of a micrometer-sized bead attached to the end of a DNA molecule has been used to deduce a strain±stress curve, which suggests a possible change in the second-order structure of the dou- ble-stranded chain [4]. The dynamic process of the condensation of single DNA molecules interacting with protamine has also been reported, where one end of DNA labeled with biotin is attached to a streptavidine-coated bead and the bead is optically trapped [8]. The key procedure in the successful manipulation of single biopolymers has been the tight attachment of the end of the polymer to a micrometer-sized object. To achieve a wider ap- plication of such single-molecular technology, it is important to manipulate macromolecules and control their conformation without any structural modi®cations. Quite recently, it has been shown that laser trapping without attachment to a mi- crometer-sized bead or any other macroscopic objects is eective for compactly folded DNA molecules [9±11]. In the present study, we have examined the optical transportation of a DNA± histone H1 complex with a continuous Neodymi- um YAG laser at 1064 nm. Histone H1 is believed to contribute to the folding of the chromatin ®ber 3 November 2000 Chemical Physics Letters 330 (2000) 77±82 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +81-52-521-2259. E-mail address: yyoshi@nagoya-bunri.ac.jp (Y. Yoshikawa). 0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 0 7 5 - 7