Telomeric DNA dimerizes by formation of guanine tetrads between hairpin loops

The telomeric ends of eukaryotic chromosomes are composed of simple repeating sequences in which one DNA strand contains short tracts of guanine residues alternating with short tracts of A/T-rich sequences. The guanine-rich strand is always oriented in a 5'-3' direction towards the end of the chromosome and is extended to produce a 3' overhang of about two repeating units in species where the telomeric terminus is known. This overhang has been implicated in the formation of several unusual intra-and intermolecular DNA structures, although none of these structures has been characterized fully. We now report that oligonucleotides encoding Tetrahymena telomeres dimerize to form stable complexes in solution. This salt-dependent dimerization is mediated entirely by the 3'-terminal telomeric overhang (TT-GGGGTTGGGG) and produces complexes in which the N7 position of every guanine in the overhangs is chemically inaccessible. We therefore propose that telomeric DNA dimerizes by hydrogen bonding between two intramolecular hairpin loops, to form antiparallel quadruplexes containing cyclic guanine base tetrads. These novel hairpin dimers may be important in telomere association and recombination and could also provide a general mechanism for pairing two double helices in other recombinational processes.     

Three-dimensional crystal structures of Escherichia coli met repressor with and without corepressor

The three-dimensional crystal structure of met repressor, in the presence or absence of bound corepressor (S-adenosylmethionine), shows a dimer of intertwined monomers, which do not have the helix-turn-helix motif characteristic of other bacterial repressor and activator structures. We propose that the interaction of met repressor with DNA occurs through either a pair of symmetry-related alpha-helices or a pair of beta-strands, and suggest a model for binding of several dimers to met operator regions.     

Regulation of the retinal interphotoreceptor matrix Na by the retinal pigment epithelium during the light response

We examined the rabbit retinal pigment epithelium (RPE) for Na transport properties which would allow it to buffer undesirable changes in Na concentrations in the interphotoreceptor matrix (IPM) during light and dark cycles. The RPE is selectivity permeable to sodium. Open and short circuit transport studies with RPE indicate a circulating (choroid to retina and back) Na current which does not compromise the electrical integrity of the blood brain barrier but together with the Na permeselectivity is of sufficient magnitude to buffer both upwards and downwards movements of IPM [Na] during light or dark responses.     

在等离子体条件下用MnO_2对铁液进行含锰还原合金化的研究

还原合金化，即当炼钢或铸造过程需要进行合金化时，不用添加铁合金，而是直接添加所需合金元素的氧化物，通过还原使其合金化。研究结果表明：在等离子体条件下，３０ｍｉｎ内可以达到合锰目标值为１２％～１８％的还原合金化。这一新的冶炼工艺特有可能从根本上改变铁合金的生产现状．     

Triggering of cyclin degradation in interphase extracts of amphibian eggs by cdc2 kinase

The cell cycles of early Xenopus embryos consist of a rapid succession of alternating S and M phases. These cycles are controlled by the activity of a protein kinase complex (cdc2 kinase) which contains two subunits. One subunit is encoded by the frog homologue of the fission yeast cdc2+ gene, p34cdc2 and the other is a cyclin. The concentration of cyclins follows a sawtooth oscillation because they accumulate in interphase and are destroyed abruptly during mitosis. The association of cyclin and p34cdc2 is not sufficient for activation of cdc2 kinase, however; dephosphorylation of key tyrosine and threonine residues of p34cdc2 is necessary to turn on its kinase activity. The activity of cdc2 kinase is thus regulated by a combination of translational and post-translational mechanisms. The loss of cdc2 kinase activity at the end of mitosis depends on the destruction of the cyclin subunits. It has been suggested that this destruction is induced by cdc2 kinase itself, thereby providing a negative feedback loop to terminate mitosis. Here we report direct experimental evidence for this idea by showing that cyclin proteolysis can be triggered by adding cdc2 kinase to a cell-free extract of interphase Xenopus eggs.     

A mutation in the tRNA(Leu)(UUR) gene associated with the MELAS subgroup of mitochondrial encephalomyopathies

Mitochondrial encephalomyopathies are usually divided into three distinct clinical subgroups: (1) mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS); (2) myoclonus epilepsy associated with ragged-red fibres (MERRF); and (3) chronic progressive external ophthalmoplegia (CPEO) including Kearns-Sayre syndrome. Large deletions of human mitochondrial DNA and a transition mutation at the mitochondrial transfer RNALys gene give rise to CPEO including Kearns-Sayre syndrome and MERRF, respectively. Here we report an A-to-G transition mutation at nucleotide pair 3,243 in the dihydrouridine loop of mitochondrial tRNA(Leu)(UUR) that is specific to patients with MELAS. Because this mutation creates an ApaI restriction site, we could perform a simple molecular diagnostic test for the disease. The mutation was present in 26 out of 31 independent MELAS patients and 1 out of 29 CPEO patients, but absent in the 5 MERRF and 50 controls tested. Southern blot analysis confirmed that the mutant DNA always coexists with the wild-type DNA (heteroplasmy).     

High levels of unintegrated HIV-1 DNA in brain tissue of AIDS dementia patients

In the host cell, retroviral DNAs exist in three main forms: unintegrated linear, unintegrated circular, and integrated (the provirus). High levels of unintegrated forms of retroviral DNA often correlate with superinfection and accompanying cytopathic effects, as, for example, in the case of feline acquired immunodeficiency. In culture, HIV-1 infection also results in high levels of unintegrated viral DNA although direct correlations with cytopathicity have not been made. The low frequency of HIV-1-infected cells in patients has made it difficult to determine the structure of the viral DNA in fresh tissue samples from AIDS patients by standard methods such as Southern hybridization. The PCR technique however, which allows the detection of viral DNA at levels far below that possible by other hybridization methods is, in its conventional form, of limited use for quantitative analysis. To study the amount and form of HIV-1 DNA in primary tissue of AIDS patients we have therefore modified the PCR method. Our results indicate that each of the three species of viral DNA are detectable in blood and brain of AIDS patients, and that in autopsy samples from patients with HIV encephalitis there is a considerably higher proportion of unintegrated viral DNA.     

Voltage-dependent InsP3-insensitive calcium channels in membranes of pancreatic endoplasmic reticulum vesicles.

Stimulus-secretion coupling in exocrine glands involves Ca2+ release from intracellular stores. In endoplasmic reticulum vesicle preparations from rat exocrine pancreas, an inositol 1,4,5-trisphosphate(InsP3)-sensitive, as well as an InsP3-insensitive, Ca2+ pool has been characterized. But Ca2+ channels in the endoplasmic reticulum of rat exocrine pancreas have not been demonstrated at the level of single-channel current. We have now used the patch-clamp technique on endoplasmic reticulum vesicles fused by means of the dehydration-rehydration method. In excised patches, single Ba2(+)- and Ca2(+)-selective channels were recorded. The channel activity was markedly voltage-dependent. Caffeine increased channel open-state probability, whereas ruthenium red and Cd2+ blocked single-channel currents. Ryanodine, nifedipine and heparin had no effect on channel activity. The channel activity was not dependent on the free Ca2+ concentration, the presence of InsP3, or pH. We conclude that this calcium channel mediates Ca2+ release from an intracellular store through an InsP3-insensitive mechanism.