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81.
Regulation of fermentative capacity was studied in chemostat cultures of two Saccharomyces cerevisiae strains: the laboratory strain CEN.PK113-7D and the industrial bakers’ yeast strain DS28911. The two strains were cultivated at a fixed dilution rate of 0.10 h−1 under various nutrient limitation regimes: aerobic and anaerobic glucose limitation, aerobic and anaerobic nitrogen limitation on glucose, and aerobic ethanol limitation. Also the effect of specific growth rate on fermentative capacity was compared in glucose-limited, aerobic cultures grown at dilution rates between 0.05 h−1 and 0.40 h−1. Biomass yields and metabolite formation patterns were identical for the two strains under all cultivation conditions tested. However, the way in which environmental conditions affected fermentative capacity (assayed off-line as ethanol production rate under anaerobic conditions) differed for the two strains. A different regulation of fermentative capacity in the two strains was also evident from the levels of the glycolytic enzymes, as determined by in vitro enzyme assays. With the exception of phosphofructokinase and pyruvate decarboxylase in the industrial strain, no clear-cut correlation between the activities of glycolytic enzymes and the fermentative capacity was found. These results emphasise the need for controlled cultivation conditions in studies on metabolic regulation in S. cerevisiae and demonstrate that conclusions from physiological studies cannot necessarily be extrapolated from one S. cerevisiae strain to the other.  相似文献   
82.
The early stages of tuber development are characterized by cell division, high metabolic activity, and the predominance of invertase as the sucrose (Suc) cleaving activity. However, during the subsequent phase of starch accumulation the cleavage of Suc occurs primarily by the action of Suc synthase. The mechanism that is responsible for this switch in Suc cleaving activities is currently unknown. One striking difference between the invertase and Suc synthase mediated cleavage of Suc is the direct involvement of inorganic pyrophosphate (PPi) in the latter case. There is presently no convincing explanation of how the PPi required to support this process is generated in potato (Solanum tuberosum) tubers. The major site of PPi production in a maturing potato tubers is likely to be the reaction catalyzed by ADP-glucose pyrophosphorylase, the first committed step of starch biosynthesis in amyloplasts. We present data based on the analysis of the PPi levels in various transgenic plants altered in starch and Suc metabolism that support the hypothesis that PPi produced in the plastid is used to support cytosolic Suc breakdown and that PPi is an important coordinator of cytosolic and plastidial metabolism in potato tubers.  相似文献   
83.
The rheology of neutrophils in their passive and activated states plays a key role in determining their function in response to inflammatory stimuli. Atomic force microscopy was used to study neutrophil rheology by measuring the complex shear modulus G*(omega) of passive nonadhered rat neutrophils on poly(HEMA) and neutrophils activated through adhesion to glass. G*(omega) was measured over three frequency decades (0.1-102.4 Hz) by indenting the cells 500 nm with a spherical tip and then applying a 50-nm amplitude multi-frequency signal. G*(omega) of both passive and adhered neutrophils increased as a power law with frequency, with a coupling between elastic (G') and loss (G') moduli. For passive neutrophils at 1.6 Hz, G' = 380 +/- 121 Pa, whereas G' was fourfold smaller and the power law coefficient was of x = 1.184. Adhered neutrophils were over twofold stiffer with a lower slope (x = 1.148). This behavior was adequately described by the power law structural damping model but not by liquid droplet and Kelvin models. The increase in stiffness with frequency may modulate neutrophil transit, arrest, and transmigration in vascular microcirculation.  相似文献   
84.
Amyloid beta protein (Abeta) is the primary constituent of plaque seen in Alzheimer's disease. Abeta is proposed to play an etiological role in Alzheimer's disease and to be a cause of the decrease in the level of acetylcholine in the hippocampus. The SAMP8 strain of mouse develops age-related increases in Abeta and deficits in learning and memory by 12 months of age. We examined in 12 month old SAMP8 mice the effects of giving antibody to Abeta by septal or intracerebroventricular (ICV) injection on acetylcholine levels in the hippocampus. Antibody to Abeta increased acetylcholine in the hippocampus over 100% after ICV injection and over 200% after septal injection. Injection of rabbit serum, antibody directed towards mouse IgG, or a blocking antibody directed towards human interleukin-1beta were without effect. These results suggest that antagonism of Abeta increases acetylcholine concentrations in the hippocampus, an area important for learning and memory.  相似文献   
85.
86.
Hinnerwisch J  Fenton WA  Furtak KJ  Farr GW  Horwich AL 《Cell》2005,121(7):1029-1041
The cylindrical Hsp100 chaperone ClpA mediates ATP-dependent unfolding of substrate proteins bearing "tag" sequences, such as the 11-residue ssrA sequence appended to proteins translationally stalled at ribosomes. Unfolding is coupled to translocation through a central channel into the associating protease, ClpP. To explore the topology and mechanism of ClpA action, we carried out chemical crosslinking and functional studies. Whereas a tag from RepA protein crosslinked proximally to the flexible N domains, the ssrA sequence in GFP-ssrA crosslinked distally in the channel to a segment of the distal ATPase domain (D2). Single substitutions placed in this D2 loop, and also in two apparently cooperating proximal (D1) loops, abolished binding of ssrA substrates and unfolded proteins lacking tags and blocked unfolding of GFP-RepA. Additionally, a substitution adjoining the D2 loop allowed binding of ssrA proteins but impaired their translocation. This loop, as in homologous nucleic-acid translocases, may bind substrates proximally and, coupled with ATP hydrolysis, translocate them distally, exerting mechanical force that mediates unfolding.  相似文献   
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88.
The Scientific Board of the California Medical Association presents the following inventory of items of progress in allergy. Each item, in the judgment of a panel of knowledgeable physicians, has recently become reasonably firmly established, both as to scientific fact and important clinical significance. The items are presented in simple epitome and an authoritative reference, both to the item itself and to the subject as a whole, is generally given for those who may be unfamiliar with a particular item. The purpose is to assist busy practitioners, students, research workers or scholars to stay abreast of these items of progress in allergy that have recently achieved a substantial degree of authoritative acceptance, whether in their own field of special interest or another.The items of progress listed below were selected by the Advisory Panel to the Section on Allergy of the California Medical Association and the summaries were prepared under its direction.  相似文献   
89.
A recent Cold Spring Harbour meeting(*) reviewed the latest progress on telomeres (the specialized structures that form the ends of chromosomes) and telomerase (the enzyme primarily responsible for their replication). Among the many aspects of telomere biology covered were strong sessions elaborating telomere replication and length regulation, telomerase structure and function, end-binding and telomere-associated proteins, DNA-damage-response proteins and telomerase-independent telomere maintenance.  相似文献   
90.
The chaperonin GroEL assists protein folding through ATP-dependent, cooperative movements that alternately create folding chambers in its two rings. The substitution E461K at the interface between these two rings causes temperature-sensitive, defective protein folding in Escherichia coli. To understand the molecular defect, we have examined the mutant chaperonin by cryo-EM. The normal out-of-register alignment of contacts between subunits of opposing wild-type rings is changed in E461K to an in-register one. This is associated with loss of cooperativity in ATP binding and hydrolysis. Consistent with the loss of negative cooperativity between rings, the cochaperonin GroES binds simultaneously to both E461K rings. These GroES-bound structures were unstable at higher temperature, dissociating into complexes of single E461K rings associated with GroES. Lacking the allosteric signal from the opposite ring, these complexes cannot release their GroES and become trapped, dead-end states.  相似文献   
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