Cloning, expression and characterization of a gene which encodes a topoisomerase I with positive supercoiling activity in pea

We have isolated and sequenced the full length cDNA for topoisomerase I. Using degenerate primers, based on the conserved amino acid sequences of five eukaryotic topoisomerase I, a 386 bp fragment was PCR amplified using pea cDNA as template. This fragment was used as a probe to screen a pea cDNA library. Two partial cDNA clones were isolated which were truncated at the 5' end. RACE-PCR was employed to isolate the remaining portion of the gene. The total size of the gene was 3055 bp with an open reading frame of 2676 bp. The deduced structure of pea topoisomerase I contain 892 amino acids with a calculated molecular weight of 100 kDa and an estimated pI of 9.3. A comparison of the deduced amino acid sequences of the pea topo I with the other eukaryotic topoisomerases clearly suggested that they are all related. Pea topoisomerase I has been overexpressed in E. coli system and the recombinant topoisomerase purified to homogeneity. The purified protein relaxes both positive and negative supercoiled DNA in the absence of divalent cation Mg2+. In the presence of Mg2+ ions the purified enzyme introduces positive supercoils a unique property not reported in any other organism except in archaebacterial topoisomerase I. Polyclonal antibodies were raised against recombinant topoisomerase I and western blotting with sub-cellular fractions indicated the localization of this topoisomerase in pea nuclei.     

抗草甘膦杂草的现状与未来展望

草甘膦-[N-(膦羧甲基)甘氨酸]是一种非选择性、广谱、内吸传导、苗后处理除草剂,三十年来广泛应用于世界各地。它抑制芳氨酸(色氨酸、酪氨酸、苯丙氨酸)生物合成,进而造成若干代谢紊乱,包括蛋白质和次生产物合成受抑制(Franz 等,1997)以及莽草酸大量积累导致代谢破坏(Duke 等,2003)。     

Novel caged clusters of silicon: Fullerenes,Frank-Kasper polyhedron and cubic

We review recent findings of metal (M) encapsulated caged clusters of Si and Ge obtained from computer experiments based on an ab initio pseudopotential method. It is shown that one M atom changes drastically the properties of Si and Ge clusters and that depending upon the size of the M atom, cages of 14, 15, and 16 Si as well as Ge atoms are formed. In particular M@Si₁₆ silicon fullerene has been obtained for M= Zr and Hf, while a Frank-Kasper polyhedron has been obtained for M@X₁₆, X = Si and Ge. These clusters show high stability and large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) gaps which are likely to make these species strongly abundant. A regular icosahedral M@X₁₂ cluster has also been obtained for X = Ge and Sn by doping a divalent M atom. Interactions between clusters are rather weak. This is attractive for developing self-assembled cluster materials.     

Theory of open circuit photo-voltage in degenerate abrupt p-n junctions

In this paper we have presented a comprehensive theory of photovoltage in degenerate abrupt p-n junction. In contrast to the earlier work[9], we use a generalized diffusion-mobility relation for charge carriers which is valid for extreme degeneracy. The general expression for the photo e.m.f. is used to obtain explicit expression for the case of lowinjection levels for two types of p−n junction. (i) When both the neutral regions are extremely degenerate and (ii) when one region is extremely degenerate but other is non-degenerate. Some numerical results of photo e.m.f. for typical p⁺n and n⁺p junctions are presented.     

Failure mechanisms in bismuth ruthenate resistor systems

Various failure mechanisms within bismuth ruthernate resistor systems associated with firing, substrate, conductor termination, laser trimming, assembly, packaging, environmental testing, power loading and high voltages, respectively, are reviewed.     

Zur Kinetik der Bildung von löslichen Polymeren beim Metathese-Abbau von C—c-vernetztem 1,4-Polybutadien

1,4-Polybutadiene was crosslinked with dicumyl peroxide. Samples with different degree of crosslinking were ground to particles with a diameter of < 0,1 mm and degraded in a metathesis reaction with a small amount of 4-octene and the catalyst WCl₆/(CH₃)₄Sn at 20°C. The fraction of the soluble polymer (soluble in chlorobenzene and precipitable by methanol) was determined as a function of reaction time and investigated by gel permeation chromatography. This fraction and in particular the higher molecular part (M > 120 000 g/mol) passes a maximum, which is more evident at a lower degree of crosslinking. The kinetics show peculiarties of the metathesis degradation and crosslinking.