Bibliothek

Notizen: Low-temperature photoluminescence experiments were carried out for Sr+ ion-implanted GaAs substrates as a function of Sr concentration and excitation intensity. It was found that the Sr impurity produces four emissions denoted by (Sr°, X), "G1'', "G2'', and "H'' in the near band emission region in GaAs grown by molecular beam epitaxy. "G1'' emission presents a red shift with increasing Sr concentration, while "G2'' does not shift. In a Sr+ ion-implanted impure GaAs substrate grown by the liquid-encapsulated Czochralski method, all four of the above emissions were missing. Instead there appeared two other emissions, SM1 and SM2, which are considered to be related to two deep acceptor levels of Sr in GaAs. All results indicate that the Sr impurity produces both shallow and deep energy levels in GaAs and a small amount of the residual impurity will quench the emissions related to shallow energy levels. © 1995 American Institute of Physics.

Notizen: An apparent transition between active and nonactive second-harmonic generation (SHG) has been discovered in the NaNb3O8 solid solutions with the tetragonal tungsten bronze structure. No SHG was detected in 0.35Na2O:Nb2O5 solid solution crystal or its powder. Nevertheless, powder SHG with a magnitude comparable to KH2PO4 was observed when a small amount of Nb2O5 was added to the powder ground from the crystal and sintered again. Microcracks and ferroelectric domain boundaries have been observed in the crystal using transmission electron microscopy. The only difference between the lattice images of SHG active powder and non-SHG active powder is the absence of microcracks and ferroelectric domain boundaries in the SHG active powder. Dielectric constant of the 0.35Na2O:Nb2O5 crystal is about 300 and δ is 0.003 for frequency from 120 kHz to 12 MHz.

Notizen: PEPT2 is functionally active and localized to the apical membrane of rat choroid plexus epithelial cells. However, little is known about the transport mechanisms of endogenous neuropeptides in choroid plexus, and the role of PEPT2 in this process. In the present study, we examined the uptake kinetics of carnosine in rat choroid plexus primary cell cultures and choroid plexus whole tissue from wild-type (PEPT2+/+) and null (PEPT2–/–) mice. Our results indicate that carnosine is preferentially taken up from the apical as opposed to basolateral membrane of cell monolayers, and that basolateral efflux in limited. Transepithelial flux of carnosine was not distinguishable from that of paracellular diffusion. The apical uptake of carnosine was characterized by a high affinity (Km = 34 μm), low capacity (Vmax = 73 pmol/mg protein/min) process, consistent with that of PEPT2. The non-saturable component was small (Kd = 0.063 μL/mg protein/min) and, under linear conditions, was only 3% of the total uptake. Studies in transgenic mice clearly demonstrated that PEPT2 was responsible for over 90% of carnosine's uptake in choroid plexus whole tissue. These findings elucidate the unique role of PEPT2 in regulating neuropeptide homeostasis at the blood–cerebrospinal fluid interface.

Notizen: Optical properties of Ca+ ion-implanted GaAs were characterized by photoluminescence measurements at 2 K. Four new emissions denoted by (Ca0,X), gCa, SM, and (e,Ca) were found to be produced by Ca incorporation into GaAs. By changing the Ca dopant concentration and excitation intensity, it was revealed that the (Ca0, X) emission is due to excitons bound to a neutral shallow Ca acceptor, and SM probably originates from excitons bound to a Ca impurity in a deep energy level. In addition, in an impure GaAs substrate, Ca could also form a shallow emission gCa, despite the fact that the g emission for well-known shallow acceptors such as C, Be, and Mg was usually found only in ultrapure GaAs. It was demonstrated that Ca is a shallow acceptor impurity in GaAs with a binding energy of 28.4 meV.

Notizen: The common bean root system is composed of several types of root (e.g. tap, basal, and lateral roots), whose physiological functions may be of great difference. However, we do not know if the root system of common bean differs in organic acid secretion and thus aluminium (Al) resistance. In the present study, the tap and basal roots of three common bean genotypes (i.e. G19842, SQ12 and BAT881) from different origins were compared for their citrate secretion and Al resistance. Grown in a simple solution containing 30 µM Al3+ for 24 h, genotype G19842 maintained 75% relative tap root length [RTRL = (tap root length with Al)/(tap root length without Al)], 48% relative basal root length [RBRL = (basal root length with Al)/ (basal root length without Al)], genotype SQ12 maintained 62% RTRL and 57% RBRL, while BAT881 only maintained 31% RTRL and 19% RBRL, indicating differential sensitivity of bean genotypes and root types to Al stress. The amounts of Al-induced citrate secretion by the tap/basal roots were 9.8/5.1, 8.2/5.9 and 5.4/4.1 nmol cm−2 FR (fresh root) [12 h]−1 for G19842, SQ12 and BAT881, respectively, indicating that both bean genotypes and root types differ in organic acid secretion. In G19842, the root surface area was 25% higher in tap root apex than that in basal root apex, and the amounts of citrate secretion per unit surface area and per root apex were 29 and 62% higher in tap root apex than those in basal root apex, respectively, suggesting that the higher citrate secretion in the tap root apex could be attributed to the larger surface area and the higher secretion activity. Stronger inhibition of Al-induced citrate secretion in the basal than tap roots by anthracene-9-carboxylic acid, an inhibitor of anion channel and K-252a, a broad range inhibitor of protein kinase may also imply the differences in the activities of anion channels and K-252a-sensitive protein kinases on the plasma membrane between the tap and basal roots, resulting in differential citrate secretion. We propose that the higher Al resistance in the tap root than in basal roots might be attributed to both greater number and higher activity of the anion channels in the former, thus allowing more citrate secretion in this root type.

Notizen: Summary: We review our studies on the mechanism of somatic hypermutation of immunoglobulin genes. Most experiments were carried out using Ig transgenes. We showed in these experiments that all required cisacting elements are present within the 10–16 kb of a cransgene. Only the Ig variable region and its proximate flanks are mutated, not the constant region. Several Ig gene enhancers are permissive for somatic mutation. Association of the enhancer with its natural Ig promoter is not necessary. However, the mutation process seems specific for Ig genes. No mutations were found in housekeeping genes from cells with high levels of somatic hypermutation of their Ig genes. The Ig enhancers may provide the Ig gene specificity. An exception may he the BCL6 gene, which was mutated in but not hut not in mouse B cellsTranscription of a region is required for its mutability When the transcriptional promoter located upstream of the variable region is duplicated upstream of the constant region, this region also becomes mutable. This suggests a model in which a mutator factor associates with the RNA polymerase at the promoter, travels with the polymerase during elongation, and causes mutations during polymerase pausing. The DNA repair systems, nucleotide excision repair and DNA mismatch repair, are not required. Our recent data with an artificial substrate of somatic mutation suggest that pausing may be due to secondary structure of the DNA or nascent RNA, and the specific mutations to preferences of the mutator factor.

Notizen: Exudation of organic anions is believed to be a common tolerance mechanism for both aluminium toxicity and phosphorus deficiency. Nevertheless, which of these stresses that actually elicit the exudation of organic anions from rape (Brassica napus L) remains unknown, and the combined effects of Al toxicity and P deficiency on rape have not been reported before. Therefore, in the current study, Brassica napus var. Natane nourin plants grown with or without 0.25 mM P were exposed to 0 or 50 µM AlCl3 and several parameters related to the exudation of organic anions from the roots were investigated. Eight days of P deficiency resulted in a significant growth reduction, but P deficiency alone did not induce exudation of organic anions. In contrast, Al strongly induced organic acid exudation, while simultaneously inhibiting root growth. Increased in-vitro activity of citrate synthase (CS, EC 4.1.3.7), malate dehydrogenase (MDH, EC 1.1.1.37) and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), together with reduced root respiration, indicated that the Al-induced accumulation and subsequent exudation of citrate and malate were associated with both increased biosynthesis and reduced metabolism of citric and malic acid. Phosphorus-sufficient plants showed more pronounced aluminium-induced accumulation and exudation of organic anions than P-deficient plants. A divided root chamber experiment showed the necessity of direct contact between Al and roots to elicit exudation of organic anions. Prolonged exposure (10 days) to Al resulted in a decrease in the net exudation of citrate and malate, and the rate of decrease was much more rapid in P-deficient plants than in P-sufficient plants. It is concluded that P nutrition affects the level of Al-induced synthesis and exudation of organic anions. However, the mechanism needs further investigation.

Notizen: Pipeline processing provides us an effective way to enhance processing speed with lowhardware costs. However, pipeline hazards are obstacles to the smooth pipelined execution ofinstructions. This paper analyzes the pipeline hazards occur in a pipeline processor designed for dataprocessing in mechanical measurements. Instruction scheduling and register renaming are performedto eliminate hazards. The simulation experiments are performed, and the effectiveness is confirmed

Notizen: This paper makes an attempt to improve the retrieving algorithm for an intelligentcommunication instrument. The instrument is developed for visually impaired computer users, and itconverts texts displayed on a computer screen into audio messages to help them operate the computer.The conversion is performed based on a user dictionary, and a retrieving algorithm is needed to searchthe dictionary. In order to accelerate the retrieving speed, we adopt an index table when retrieving thefirst two alphabets of a word in the dictionary. The time for retrieving words is measured in theexperiments. The experimental results reveal that the retrieving operations can be finished within anacceptable time