Formation and resolution of recombination intermediates by E. coli RecA and RuvC proteins.

The recombination of DNA molecules has been reconstituted in vitro using two purified enzymes from Escherichia coli. RecA protein catalyses homologous pairing and strand exchange reactions to form intermediate DNA structures that are acted upon by RuvC. The newly identified RuvC protein resolves the intermediates by specific endonucleolytic cleavage to produce recombinant DNA molecules.     

建立以"语言学习"为中心的英语导学新模式

开放教育(中国电大系统试行的一种远距离教育)打破了传统的教学模式,带来了开放学员学习信心和学习效率如何提高等一系列问题.而学习中心论提供了英语语言开放教育与传统语言教育的最佳结合点.作者认为,依据学习中心论可以最大限度地调动开放学员的英语语言认知积极性,从而可以使开放教育中的成人学员在英语语言学习方面的障碍得到很好的解决.     

An infrared flash contemporaneous with the gamma-rays of GRB 041219a

The explosion that results in a cosmic gamma-ray burst (GRB) is thought to produce emission from two physical processes: the central engine gives rise to the high-energy emission of the burst through internal shocking, and the subsequent interaction of the flow with the external environment produces long-wavelength afterglows. Although observations of afterglows continue to refine our understanding of GRB progenitors and relativistic shocks, gamma-ray observations alone have not yielded a clear picture of the origin of the prompt emission nor details of the central engine. Only one concurrent visible-light transient has been found and it was associated with emission from an external shock. Here we report the discovery of infrared emission contemporaneous with a GRB, beginning 7.2 minutes after the onset of GRB 041219a (ref. 8). We acquired 21 images during the active phase of the burst, yielding early multi-colour observations. Our analysis of the initial infrared pulse suggests an origin consistent with internal shocks.     

Transdermal uptake of a peptide hormone: inhibition by calcitonin eardrops of induced osteolysis in guinea-pig ossicles.

In view of a recent proposal that calcitonin injections may arrest the bony pathology of otosclerosis, we have tested the possibility of obtaining locally effective concentrations by giving salmon calcitonin in eardrops. Osteolysis of guinea-pig ossicles induced by injecting parathyroid hormone shortly before explantation was markedly inhibited by 3 days prior instillation of the calcitonin in an aqueous vehicle or in dimethyl sulphoxide, but not by a solution in propylene carbonate.     

Influence of continuous light or darkness on sexual development in the male rabbit

Zusammenfassung Gruppen von männlichen Kaninchen verschiedenen Alters wurden in verschiedenen Jahreszeiten unter Dauerbelichtung, Dauerverdunkelung und unter natürlichen Lichtverhältnissen gehalten, und zwar jeweilen während einer Periode von 4, 8 und 20 Wochen. Es ergaben sich keine signifikanten Unterschiede, weder im Gewicht der Testes und der akzessorischen Geschlechtsorgane noch in ihrer histologischen Struktur, noch im Fruktosegehalt des Samenblasensekrets zwischen den Versuchstieren und den Kontrollen.     

Antarctic climate cooling and terrestrial ecosystem response.

The average air temperature at the Earth's surface has increased by 0.06 degrees C per decade during the 20th century, and by 0.19 degrees C per decade from 1979 to 1998. Climate models generally predict amplified warming in polar regions, as observed in Antarctica's peninsula region over the second half of the 20th century. Although previous reports suggest slight recent continental warming, our spatial analysis of Antarctic meteorological data demonstrates a net cooling on the Antarctic continent between 1966 and 2000, particularly during summer and autumn. The McMurdo Dry Valleys have cooled by 0.7 degrees C per decade between 1986 and 2000, with similar pronounced seasonal trends. Summer cooling is particularly important to Antarctic terrestrial ecosystems that are poised at the interface of ice and water. Here we present data from the dry valleys representing evidence of rapid terrestrial ecosystem response to climate cooling in Antarctica, including decreased primary productivity of lakes (6-9% per year) and declining numbers of soil invertebrates (more than 10% per year). Continental Antarctic cooling, especially the seasonality of cooling, poses challenges to models of climate and ecosystem change.     

Depletion of synaptic vesicles at the frog (Rana pipiens) neuromuscular junctions by tetraphenylboron

Summary At the frog neuromuscular junction tetraphenylboron produced a decrease in miniature endplate potential amplitude, accompanied by a decrease in the number but not the size of synaptic vesicles.This work was supported by NIH Grants Nos NS-07740 and NS-05707 to R. L. P. and was done during the tenure of a Research Fellowship of Muscular Dystrophy Associations of America, a Wellcome Research Travel Grant and a grant from the Carnegie Trust for the Universities of Scotland to I. G. M. The assistance of Dr.R. M. Schnitzler, W. G. Boldosser andP. M. Spannbauer is gratefully acknowledged.