Sub-exponential growth and coexistence of non-enzymatically replicating templates

The experimentally reported kinetic behaviour (sub-exponential but supra-linear growth) of non-enzymatic template replication is incorporated into a simple model of template competition. Sub-exponential growth is shown to lead to coexistence invariably. Thus coexistence of different non-enzymatically replicating sequences is predicted. This type of coexistence could have been important in maintaining a sufficient diversity of RNA modules used later to build functional molecules such as ribozymes. Experimental tests of this theoretical prediction are possible.     

The effect of a random initial value in neural first-passage-time models

The effect of a random initial value is examined in several stochastic integrate-and-fire neural models with a constant threshold and a constant input. The three models considered are approximations of Stein's model, namely: (1) a leaky integrator with deterministic trajectories, (2) a Wiener process with drift, and (3) an Ornstein-Uhlenbeck process. For model 1, different distributions for the initial value lead to commonly observed interspike interval distributions. For model 2, a discrete and a uniform distribution for the initial value are examined along with some parameter estimation procedures. For model 3, with a truncated normal distribution for the initial value, the coefficient of variation is shown to be greater than 1, and as the threshold becomes large the first-passage-time distribution approaches an exponential distribution. The relationships among the models and between them and previous models are also discussed, along with the robustness of the model assumptions and methods of their verification. The effects of a random initial value are found to be most pronounced at high firing rates.     

Geometric optics of trilobite eyes: a theoretical study of the shape of the aspherical interface in the cornea of schizochroal eyes of phacopid trilobites

A general method is given to determine theoretically the shape of the aspherical interface that eliminates spherical aberration in an optionally shaped thick lens. The theory is applied to trilobite eyes. On the basis of the geometric optical method presented, the refractive indices and focal length of the original corneal lenses of trilobites can be determined. The shape of the aspherical interface in the cornea of some phacopid trilobites with schizochroal eyes is investigated. The theoretical aspherical interfaces agree well with the real ones.     

Fine needle aspiration cytology of primary soft tissue tumors. Morphologic analysis of the most frequent types.

We reviewed 98 cases of fine needle aspiration of soft tissue tumors with histologic confirmation performed during an eight-year period and propose a working morphologic classification based on the most prominent cytologic features. Six main tumor groups are recognized: myxoid, round cells, spindle cells, pleomorphic cells, polygonal cells and well differentiated. We believe that this classification allows recognition of the most common soft tissue tumors while helping with the differential diagnosis of other neoplasia, primary or secondary, with similar morphology and a similar presentation.     

Fine needle aspiration of peripheral neuroepithelioma of soft tissues.

Peripheral neuroepithelioma of soft tissue is a malignant primitive neuroectodermal tumor that appears both in children and adults and usually has a poor outcome. Fine needle aspiration on two patients with tumors in the lower limbs showed small round cells with unipolar processes and a tendency to form Homer-Wright rosettes. The cells had a round to oval nucleus with fine chromatin, up to four small, conspicuous nucleoli and vacuolated, periodic acid-Schiff-positive cytoplasm. The diagnosis was supported by electron microscopic study of the aspirate, which showed features of neuroblastic differentiation (i.e., neurosecretory granules), and by histologic, immunohistochemical and cytogenetic study of the resected tumors.     

Isolation of peroxisomes from frozen human liver samples.

This paper shows the successful isolation of peroxisomes from human liver samples that were kept frozen at -70 degrees C. Purification of these peroxisomes was obtained by a combination of two subcellular fractionation techniques: differential centrifugation and isopycnic fractionation in Nycodenz density gradients. Peroxisome integrity was evaluated by latency measurements and by ultrastructural observation. The procedure described here may be useful for the isolation of other subcellular organelles from frozen human samples.     

Plasmid-determined resistance to arsenic and antimony inPseudomonas aeruginosa

Resistance to arsenic salts in aPseudomonas aeruginosa clinical isolate was shown to be determined by a 100 kb transferable plasmid. The resistance pattern included arsenate, arsenite, and antimonate ions. Arsenate and arsenite resistances were inducible by previous exposure of cultures to subinhibitory amounts of either of the two ions. Phosphate ions protectedP. aeruginosa cells from the toxic effects of arsenate but did not alter arsenite toxicity.     

Purification and characterization of glutathione reductase from Rhodospirillum rubrum.

Glutathione reductase (NAD(P)H:GSSG oxidoreductase EC 1.6.4.2.) was purified 1160-fold to homogeneity from the nonsulfurous purple bacteria Rhodospirillum rubrum (wild type). Specific activity of the pure preparation was 102 U/mg. The enzyme displayed a typical flavoprotein absorption spectrum with maxima at 274,365, and 459 nm and an absorbance ratio A280/A459 of 7.6. The amino acid analysis revealed an unusually high content of glycine and arginine residues. Titration of the enzyme with 5,5'-dithiobis(2-nitrobenzoic acid) showed a total of two free thiol groups per subunit, one of which is made accessible only under denaturing conditions. An isoelectric point of 5.2 was found for the native enzyme. Km values, determined at pH 7.5, were 6.1 and 90 microM for NADPH and GSSG, respectively. NADH was about 2% as active as NADPH as an electron donor. The enzyme's second choice in disulfide substrate was the mixed disulfide of coenzyme A and glutathione, for which the specific activity and Km values were 5.1 U/mg and 3.4 mM, respectively. A native molecular weight of 118,000 was found, while denaturing electrophoresis gave a value of 54,400 per subunit, thus suggesting that R. rubrum glutathione reductase exists as a dimeric protein. Other physicochemical constants of the enzyme, such as Stokes radius (4.2 nm) and sedimentation coefficient (5.71 S), were also consistent with a particle of 110,000.     

Characteristics of NADPH-dependent thymidylate synthetase purified from Streptomyces aureofaciens.

NADPH-dependent thymidylate synthetase from Streptomyces aureofaciens has been purified to homogenity by a two-step chromatographic procedure including anion-exchange chromatography and affinity chromatography on methotrexate-Sepharose 4B. The enzyme was purified 1025-fold with a 34% yield. Basic characteristics of the enzyme were determined: molecular weight of the enzyme subunit (28,000), pH and temperature optimum, effect of cations, dependency on reducing agents, Km values for dUMP, mTHF, and NADPH (3.78, 21.1, and 38.9 microM, respectively), and inhibition effect of 5-FdUMP. Binding studies revealed the enzyme mechanism to be ordered sequential: dUMP bound before mTHF. S. aureofaciens thymidylate synthetase exhibits an absolute requirement for NADPH for the enzyme activity--a unique feature not displayed by any of the thymidylate synthetases isolated so far. NADPH is not consumed during enzyme reaction, indicating its regulatory role. The properties of S. aureofaciens thymidylate synthetase show that it is a monofunctional bacterial enzyme.     

Two novel monoclonal antibodies to fibronectin that recognize the Hep II and CS-1 regions respectively: their differential effect on lymphocyte adhesion.

We have obtained two new mAbs to the carboxy-terminal region of fibronectin, namely P3D4 and P1F11, and have studied their binding sites and their ability to block lymphocyte adhesion to fibronectin. ELISA and Western blot analyses showed that P3D4 reacts with both fibronectin chains and both Hep II-containing fragments (58 kDa and 38 kDa). P1F11, raised against the synthetic peptide CS-1, reacted with the 38 kDa fragment and with a 190 kDa fragment derived from the A chain of fibronectin. P1F11 did not react with the 58 kDa fragment thus clearly establishing that 58 kDa comes from the B chain of fibronectin and lacks the CS-1 sequence. mAbs P3D4 and P1F11 were used to evaluate the contribution of the Hep II and CS-1 sites in cell attachment to fibronectin. P3D4 effectively inhibited B cell adhesion to 38 kDa, 58 kDa and fibronectin; P1F11 however produced only limited inhibition, suggesting that lymphocyte interaction with Hep II may modulate further binding to the CS-1 site.