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1.
We carried out experiments on single isolated myofibrils in which thin filaments had been functionally removed, leaving the connecting (titin) filaments as the sole agent taking up the length change. With technical advances that gave sub-nanometer detectability we examined the time course of single sarcomere-length change when the myofibril was ramp-released or ramp-stretched by a motor. The sarcomere-length change was stepwise. Step sizes followed a consistent pattern: the smallest was approximately 2.3 nm, and others were integer multiples of that value. The approximately 2.3-nm step quantum is the smallest consistent biomechanical event ever demonstrated. Although the length change must involve the connecting filament, the size of the quantum is an order of magnitude smaller than anticipated from folding of Ig- or fibronectin-like domains, implying either that folding occurs in sub-domain units or that other mechanisms are involved. 相似文献
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3.
It has been reported that Candida albicans can form germ-tubes only in the narrow pH range of 6-8, and that by changing only the pH one can regulate germ-tube formation. We found that the pH minimum for germ-tube formation could be dramatically lowered by eliminating the glucose present in many induction solutions. Lee's medium lacking glucose, ethanol, N-acetyl-D-glucosamine, and proline induced germ-tubes at pH values as low as 3 under most conditions. The presence of as little as 1 mM-glucose in these induction solutions was sufficient to cause the cells to grow either as yeasts with multiple buds or as pseudohyphae when the pH was 3.7. However, when C. albicans was grown in any of the above induction solutions (with the exception of ethanol), containing 200 mM-glucose buffered at pH 5.8, not only were germ-tubes formed, but their rate of formation and length were also increased. Preincubation of the cells in a solution buffered at pH 3.7 and containing 200 mM-glucose, before exposure to induction solutions lacking glucose at pH 3.7 or at pH 5.8, did not inhibit germ-tube formation. Likewise, addition of glucose after 45 min exposure to an induction solution was without effect. Theophylline and dibutyryl cAMP did not counteract the action of glucose. Other sugars which suppressed germ-tube formation at low pH were fructose, galactose, mannose, xylose, gluconic acid and the nonmetabolizable sugar 3-O-methylglucose. These results indicate that pH does not directly regulate dimorphism in C. albicans, and that glucose or its metabolites may play an important role. 相似文献
4.
Assignment of autosomal dominant spinocerebellar ataxia (SCA1) centromeric to the HLA region on the short arm of chromosome 6, using multilocus linkage analysis. 总被引:6,自引:6,他引:0
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H Y Zoghbi L A Sandkuyl J Ott S P Daiger M Pollack W E O''''Brien A L Beaudet 《American journal of human genetics》1989,44(2):255-263
A 7-generation kindred with the HLA-linked form of spinocerebellar ataxia (SCA1) was studied to determine whether the SCA1 gene maps centromeric or telomeric to the HLA loci. The DNA markers flanking the HLA-(A-B) region were used for polymorphism studies and multilocus linkage analysis. These two markers are the cDNA for the beta-subunit of HLA-DP, which is centromeric to HLA-(A-B), and the cDNA for coagulation factor XIIIa (F13A), which is telomeric to HLA-(A-B). Haplotypes were constructed using multiple polymorphisms for these two DNA markers, and pairwise linkage analysis revealed a maximum lod score of 2.18 for SCA1 versus HLA-DP at a recombination fraction of .05 and a maximum lod score of 0 for SCA1 versus F13A at a recombination fraction of .50. A possible crossover between HLA-(A-B) and HLA-DP was identified, but lack of samples from key individuals hampered the analysis. To clarify the phase and improve the analysis, the two chromosomes 6 for the crossover individual were separated in somatic cell hybrids. The results strongly favored the probability that the crossover occurred between HLA-(A-B-DR) and HLA-DP with SCA1 segregating with HLA-DP, consistent with a location centromeric to HLA-(A-B). Multilocus linkage analysis was used to evaluate further the location of SCA1 relative to F13A, HLA-(A-B), and HLA-DP; the results indicated that the SCA1 gene locus is centromeric to HLA-DP with odds of 46:1 favoring this most likely location over the second most likely location, i.e., telomeric to HLA-(A-B) between the HLA complex and F13A. 相似文献
5.
Summary The development of the sensory neurons of the legs of the blowfly,Phormia regina has been described from the third instar larva to the late pupa using immunohistochemical staining. The leg discs of the third instar larva contain 8 neurons of which 5 come to lie in the fifth tarsomere of the developing leg. Whereas 2 neurons persist at least to the late pupa, the other cells degenerate. The first neurons of gustatory sensilla arise in the fifth tarsomere at about 1.5 h after formation of the puparium. Most of these sensilla, however, appear within a short time period beginning at about 18 h. The femoral chordotonal sensory neurons first appear at the time of formation of the puparium, as a mass of cells situated in the distal femur. During later pupal development 2 groups of these cells come to lie at the femur-trochanter border, where they become the proximal femoral chordotonal organ of the adult; the remaining cells become the distal femoral chordotonal organ. Other scolopidial neurons appear later in development. The nerve pathways of the late pupal leg are established either by the axons of the cells that are present in the larval leg disc or by new outgrowing processes of sensory neurons. In the tibia, the initial direction of new outgrowth differs in different regions of the segment: proximal tibial neurons grow distally, while distal tibial neurons grow initially proximally. 相似文献
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7.
A comparison of the escape behaviors of the cockroaches Blaberus craniifer and Periplaneta americana
A comparison of wind-mediated escape behavior of two species of cockroaches revealed striking differences. In contrast to Periplaneta americana, Blaberus craniifer was unable to escape from a live predator or move away from a synthetically generated puff of wind. Other behavioral differences included degree of preference for areas of low light intensity and propensity to dig and thus bury oneself when disturbed. Anatomical differences, although present, did not seem sufficient to account for the behavioral differences. Also, B. craniifer was able to escape from generated wind puffs but not a live predator when its temperature was raised. These points suggest that the behavioral differences in escape behavior can be attributed to differences in the physiological state of the two nervous systems. 相似文献
8.
A photosynthetic apparatus is present in the epidermis of the bottom zone of the pitcher of Sarracenia purpurea L. ssp. purpurea. This has been demonstrated using conventional light and electron microscopy, as well as fluorescent and immunohistochemical techniques. Red intrinsic fluorescence by these chloroplasts indicates photochemical activity. Antibodies against the coupling factor of chloroplast ATPase and against the subunits of ribulose-bis-phosphate-carboxylase were bound to the epidermal chloroplasts. This has been visualized using a ferritin-isothiocyanate labeled second antibody. These results unequivocally prove the existence of the two main proteins which are associated with the photophosphorylation (membrane protein) and carbon dioxide fixation (stromal protein). The possible implication of this system to interrelationships between the carnivorous plant and aquatic insects inhabiting its leaves is discussed. 相似文献
9.
Purine metabolism in Acholeplasma laidlawii B: novel PPi-dependent nucleoside kinase activity 总被引:5,自引:5,他引:0
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Acholeplasma laidlawii B-PG9 was examined for 16 cytoplasmic enzymes with activity for purine salvage and interconversion. Phosphoribosyltransferase activities for adenine, guanine, xanthine, and hypoxanthine were shown. Adenine, guanine, xanthine, and hypoxanthine were ribosylated to their nucleoside. Adenosine, inosine, xanthosine, and guanosine were converted to their base. No ATP-dependent phosphorylation of nucleosides to mononucleotides was found. However, PPi-dependent phosphorylation of adenosine, inosine, and guanosine to AMP, inosine monophosphate, and GMP, respectively, was detected. Nucleotidase activity for AMP, inosine monophosphate, xanthosine monophosphate, and GMP was also found. Interconversion of GMP to AMP was detected. Enzyme activities for the interconversion of AMP to GMP were not detected. Therefore, A. laidlawii B-PG9 cannot synthesize guanylates from adenylates or inosinates. De novo synthesis of purines was not detected. This study demonstrates that A. laidlawii B-PG9 has the enzyme activities for the salvage and limited interconversion of purines and, except for purine nucleoside kinase activity, is similar to Mycoplasma mycoides subsp. mycoides. This is the first report of a PPi-dependent nucleoside kinase activity in any organism. 相似文献
10.
Light inhibits while carbon dioxide enhances the conversion of 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene in oat ( Avena sativa L. cv. Victory) leaf segments. The possibility that the light inhibition is mediated through changes of carbon dioxide has been investigated. The level of CO2 increases or decreases in the sealed incubation vial in darkness or in light, respectively, which can apparently account for the differences in ACC-dependent ethylene production between the dark and light treatments. However, although the evolution of ethylene from ACC in the dark is reduced upon depletion of CO2 , the difference between light and dark is still very noticeable. Moreover, the production of the ethylene in CO2 -free air in the dark was still higher than in the light, where the concentration of CO2 was 0.01%. It is proposed that the light effect on the conversion of ACC to ethylene is composed of two distinguishable components: one CO2 -mediated and the other CO2 -independent. 相似文献