Mitochondrial heat-shock protein hsp60 is essential for assembly of proteins imported into yeast mitochondria

A nuclear encoded mitochondrial heat-shock protein hsp60 is required for the assembly into oligomeric complexes of proteins imported into the mitochondrial matrix. hsp60 is a member of the 'chaperonin' class of protein factors, which include the Escherichia coli groEL protein and the Rubisco subunit-binding protein of chloroplasts.     

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.     

胺钨盐的性质及其在制取亚微米级钨粉方面的应用

本文着重研究了自还原性胺钨盐的性质及其氢还原,发现,将自还原与一般氢还原适当结合,可以高效率地制备比表面积大于20m~2/g的极细、超细钨粉。对自还原的本性及该类钨盐在制取极细、超细钨粉方面的应用前景进行了讨论。     

Loss of photosynthetic and chlororespiratory genes from the plastid genome of a parasitic flowering plant

Photosynthesis is the hallmark of plant life and is the only plastid metabolic process known to be controlled by plastid genes. The complete loss of photosynthetic ability, however, has occurred on several independent occasions in parasitic flowering plants. Some of these plants are known to lack chlorophyll and certain photosynthetic enzymes, but it is not known to what extent changes have occurred in the genes encoding the photosynthetic apparatus or whether the plants even maintain a plastid genome. Here we report that the nonphotosynthetic root parasite Epifagus virginiana has a plastid chromosome only 71 kilobases in size, far smaller than any previously characterized land plant plastid genome. The Epifagus plastid genome has lost most, if not all, of the 30 or more chloroplast genes for photosynthesis and most of a large family of plastid genes, the ndh genes, whose products may be involved in a plastid respiratory chain. The extensive changes in Epifagus plastid gene content must have occurred in a relatively short time (5-50 x 10(6) yr), because Striga asiatica, a related photosynthetic parasite, has a typical complement of chloroplast genes for photosynthesis and chlororespiration. The plastid genome of Epifagus has retained transcribed ribosomal RNA and ribosomal protein genes, suggesting that it expresses one or more gene products for plastid functions not related to photosynthesis.     

Regulation and deregulation of cardiac Na(+)-Ca2+ exchange in giant excised sarcolemmal membrane patches

A plasmalemmal Na(+)-Ca2+ exchange mechanism is an important electrogenic determinant of contractility in cardiac cells. As in other cell types, calcium influx by Na(+)-Ca2+ exchange is secondarily activated by cytoplasmic calcium and probably ATP, but these modulatory mechanisms are either absent or altered in isolated cardiac sarcolemmal vesicles. Involvement of a calcium-dependent protein kinase in exchange regulation has been suggested but not verified. Here I describe measurements of outward Na(+)-Ca2+ exchange current, corresponding to calcium influx, in giant excised sarcolemmal patches from guinea pig myocytes. The exchange current is stimulated by both calcium and Mg-ATP from the cytoplasmic face, evidently through separate mechanisms. Activation by cytoplasmic calcium takes place within seconds, is reversible, and does not require ATP. Stimulation by Mg-ATP reverses only slowly over greater than 10 min, or not at all. Unexpectedly, a substantial decrease in exchange current occurs during activation by cytoplasmic sodium, which seems to reflect an inactivation process rather than ion concentration changes or a 'first pass' exchange cycle. This apparent inactivation, and the modulations by cytoplasmic calcium and Mg-ATP, are all abolished by brief treatment of the cytoplasmic surface with chymotrypsin, leaving the exchanger in a maintained state of high activity. Therefore, limited proteolysis deregulates Na(+)-Ca2+ exchange and could contribute to the loss of secondary regulation of the exchange in isolated sarcolemmal vesicles.     

Residues of the variable region of the T-cell-receptor beta-chain that interact with S. aureus toxin superantigens

The alpha beta T-cell antigen receptor (TCR) recognizes antigenic peptides in the context of self major histocompatibility complex (MHC) molecules. The specificity of recognition of MHC plus antigen is generally determined by a combination of the variable elements of alpha- and beta-chains of the TCR. Several types of antigen, however, have been identified that, when bound to MHC molecules, stimulate T cells bearing particular variable-region beta-chain (V beta) elements irrespective of the other variable components of the TCR. These have been termed 'superantigens', and here we are concerned with one type of superantigen, the toxins produced by Staphylococcus aureus. T cells have been found that bear closely related members of the same V beta family but respond differently to S. aureus toxins; in particular, cells bearing the human V beta 13.2 element respond to toxin SEC2, whereas cells bearing human V beta 13.1 do not. We have now defined the residues of the V beta element responsible for this difference, and find that they reside in a region thought to lie on the side of the TCR molecule, away from the conventional antigen/MHC-binding site. The evolutionary conservation of this site may be due to its having an important role in some function of the TCR other than the binding of conventional antigen plus MHC.     

Expression of cystic fibrosis transmembrane conductance regulator corrects defective chloride channel regulation in cystic fibrosis airway epithelial cells.

The cystic fibrosis transmembrane conductance regulator (CFTR) was expressed in cultured cystic fibrosis airway epithelial cells and Cl- channel activation assessed in single cells using a fluorescence microscopic assay and the patch-clamp technique. Expression of CFTR, but not of a mutant form of CFTR (delta F508), corrected the Cl- channel defect. Correction of the phenotypic defect demonstrates a causal relationship between mutations in the CFTR gene and defective Cl- transport which is the hallmark of the disease.     

Expression and characterization of the cystic fibrosis transmembrane conductance regulator.

Cystic fibrosis (CF) is a common lethal genetic disease that manifests itself in airway and other epithelial cells as defective chloride ion absorption and secretion, resulting at least in part from a defect in a cyclic AMP-regulated, outwardly-rectifying Cl- channel in the apical surface. The gene responsible for CF has been identified and predicted to encode a membrane protein termed the CF transmembrane conductance regulator (CFTR). Identification of a cryptic bacterial promoter within the CFTR coding sequence led us to construct a complementary DNA in a low-copy-number plasmid, thereby avoiding the deleterious effects of CFTR expression on Escherischia coli. We have used this cDNA to express CFTR in vitro and in vivo. Here we demonstrate that CFTR is a membrane-associated glycoprotein that can be phosporylated in vitro by cAMP-dependent protein kinase. Polyclonal and monoclonal antibodies directed against distinct domains of the protein immunoprecipitated recombinant CFTR as well as the endogenous CFTR in nonrecombinant T84 cells. Partial proteolysis fingerprinting showed that the recombinant and non-recombinant proteins are indistinguishable. These data, which establish several characteristics of the protein responsible for CF, will now enable CFTR function to be studied and will provide a basis for diagnosis and therapy.     

Different voltage-dependent thresholds for inducing long-term depression and long-term potentiation in slices of rat visual cortex

In the hippocampus and neocortex, high-frequency (tetanic) stimulation of an afferent pathway leads to long-term potentiation (LTP) of synaptic transmission. In the hippocampus it has recently been shown that long-term depression (LTD) of excitatory transmission can also be induced by certain combinations of synaptic activation. In most hippocampal and all neocortical pathways studied so far, the induction of LTP requires the activation of N-methyl-D-aspartate (NMDA) receptor-gated conductances. Here we report that LTD can occur in neurons of slices of the rat visual cortex and that the same tetanic stimulation can induce either LTP or LTD depending on the level of depolarization of the postsynaptic neuron. By applying intracellular current injections or pharmacological disinhibition to modify the depolarizing response of the postsynaptic neuron to tetanic stimulation, we show that the mechanisms of induction of LTD and LTP are both postsynaptic. LTD is obtained if postsynaptic depolarization exceeds a critical level but remains below a threshold related to NMDA receptor-gated conductances, whereas LTP is induced if this second threshold is reached.