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981.
The Vaccinia type I topoisomerase catalyzes site-specific DNA strand cleavage and religation by forming a transient phosphotyrosyl linkage between the DNA and Tyr-274, resulting in the release of DNA supercoils. For type I topoisomerases, two mechanisms have been proposed for supercoil release: (I) a coupled mechanism termed strand passage, in which a single supercoil is removed per cleavage/religation cycle, resulting in multiple topoisomer intermediates and late product formation, or (2) an uncoupled mechanism termed free rotation, where multiple supercoils are removed per cleavage/religation cycle, resulting in few intermediates and early product formation. To determine the mechanism, single-turnover experiments were done with supercoiled plasmid DNA under conditions in which the topoisomerase cleaves predominantly at a single site per DNA molecule. The concentrations of supercoiled substrate, intermediate topoisomers, and relaxed product vs time were measured by fluorescence imaging, and the rate constants for their interconversion were determined by kinetic simulation. Few intermediates and early product formation were observed. From these data, the rate constants for cleavage (0.3 s(-1)), religation (4 s(-1)), and the cleavage equilibrium constant on the enzyme (0.075) at 22 degrees C are in reasonable agreement with those obtained with small oligonucleotide substrates, while the rotation rate of the cleaved DNA strand is fast (approximately 20 rotations/s). Thus, the average number of supercoils removed for each cleavage event greatly exceeds unity (delta n = 5) and depends on kinetic competition between religation and supercoil release, establishing a free rotation mechanism. This free rotation mechanism for a type I topoisomerase differs from the strand passage mechanism proposed for the type II enzymes. 相似文献
982.
983.
984.
985.
986.
Hsien-Chin Chin Ming-Jyh Hwu Shih-Cheng Yang Yi-Jen Chan 《Electron Device Letters, IEEE》2002,23(5):243-245
Surface passivation technology plays an important role, especially in E-mode pHEMTs applications, and a new passivation technology has been proposed in this study. This novel benzocyclobutene (BCB) passivation layer takes advantage of the low dielectric permittivity (2.7) and a low loss tangent (0.0008). In this letter, we not only suppress the gate-to-drain leakage current but also improve the device power performance under a high input power swing by using a BCB passivation layer. The passivated 1.0 μm-long gate pHEMTs exhibit a better off-state performance than the unpassivated ones. The maximum output power under a 2.4-GHz operation is 118 mW/mm, with a linear power gain of 11.1 dB and a power-added efficiency is 60% 相似文献
987.
Shengdong Zhang Ruqi Han Zhikuan Zhang Ru Huang Ko P.K. Mansun Chan 《Electron Device Letters, IEEE》2002,23(10):618-620
This letter reports the implementation of a bottom-gate MOSFET, which possesses the following fully self-aligned structural features: 1) self-aligned source-drain to bottom-gate; 2) self-aligned thick source-drain and thin channel; and 3) self-aligned and mask-free lightly doped drain (LDD). The complete self-alignment is realized by combining a conventional ion implantation and a subsequent chemical-mechanical polishing (CMP) step. The process is applied to poly-Si films crystallized from an a-Si film deposited by LPCVD using a metal-induced unilateral crystallization technique, and is grain-enhanced further in a high temperature annealing step. Deep submicron fully self-aligned bottom-gate pMOS transistors with channel length less than 0.5 /spl mu/m are fabricated. The measured performance parameters include threshold voltage of -0.43 V, subthreshold swing of 113 mV/dec, effective hole mobility of 147 cm/sup 2//V-s, off-current of 0.17 pA//spl mu/m, and on-off current ratio of 7.1/spl times/10/sup 8/. 相似文献
988.
A representation or model of a digital magnetic recording channel which has value both as an expression of the transfer characteristics of the channel and as a tool for use in the design and development of digital coding techniques for that channel is presented 相似文献
989.
Seung Jai Kim Chan Yong Chung Sung Yong Cho Rok Ju Chang 《Korean Journal of Chemical Engineering》1991,8(2):73-79
Rates of mass transfer from the liquid phase to small ion exchange resin particles (0.78 mm in mean diameter) in fluidized
and spouted beds were studied experimentally. Dilute aqueous solution of sodium hydroxide was fed into the beds of strong
cation exchange resin and the exit concentration of the solution was determined by conductivity measurement. In spouted beds,
the initial conversion and Kl increased with bed height, but decreased with fluid flowrate. The model, applying material balance of the reactant and axisymmetric
flow of fluid in the annulus of a spouted bed, predictions of the initial conversion in spouted beds are satisfactory. In
fluidized beds, the obtained mass transfer coefficients were correlated and compared with other works. 相似文献
990.