Boundaries of the universal K3 region and plateau region of the dynamic structure factor for DNA

A sufficiently long semiflexible filamentous macromolecule is theroretically expected to exhibit three different domains of behavior of its apparent diffusion coefficient D_app(K) as a function of scattering vector K: (1) the small wave vector limit, where D_app(K) = D₀ is the translational diffusion coefficient of the center-of-mass; (2) the universal K³ region, where D_app(K) = (k_BT/6πη)K is a universal function of K independent of any property of the molecule itself; (c) the plateau region at large K², where D_app(K) approaches either a plateau, or gradually sloping quasiplateau, characteristic of local (elastic) rigid-body motions of the filament. The existence of each of these different domains has now been established experimentally for at least some polymers. The boundaries of the universal K³ region and the plateau region are determined theoretically here using precise quantitative criteria for universal or plateau behavior of D_app(K) for a Rouse-Zimm model containing N + 1 subchains with rms subchain extension b. Allowing a maximum of 13% nonuniversal behavior, the domain of the universal K³ region is given by K²R²_G = K²Nb²/6 ≥ 7 and K²b² ? 0.54. Allowing as much as 10% nonplateau behavior, the boundary for onset of plateau behavior is K²b² = 18.3. D_app(K) is at least 50% nonuniversal when K²b²/6 = 6 ln 3. Extension of these results to DNA is examined theoretically, and good agreement of the pertinent predictions with published experimental data is demonstrated. It is concluded that no truly universal K³ region exists for DNA with M_r ? 10⁷ and persistence length a ≥ 450 Å, although marginally (?17% nonuniversal) universal behavior, is exhibited in a very narrow domain 0.64 × 10¹⁰ ? K² ? 0.84 × 10¹⁰ cm^?2 for ?29 DNA (M_r = 11.5 × 10⁶). More than 50% of D_app(K) is governed by local (elastic) rigid-body motions when K² = 5.23 × 10¹⁰ cm^?2. The existence of a very wide region of nonuniversal apparent K³ behavior extending up to very large K², far into the plateau region, is demonstrated in a plot of D_app(K)/K vs K² for the Rouse-Zimm model. This is shown to stem in part from visual artifacts of plotting D_app(K)/K vs K², even for rigid species.     

Temperature dependence of the dynamic light scattering of linear phi 29 DNA: implications for spontaneous opening of the double-helix

The apparent diffusion coefficient D_app(K) of a single sample of linear ?29 DNA (M_r = 11.5 × 10⁶) has been measured as a function of K² from 0.21 × 10¹⁰ to 20 × 10¹⁰ cm^?2 at a variety of temperatures from ?0.5 to +70°C. D_app(K) scales closely as T/η at every value of K². All of these data are simulated by a particular Rouse-Zimm model comprised of a constant number of subchains with constant rms subchain extension b = 1057 Å and an apparent subchain diffusion coefficient D_plat that scales at T/η from ?0.5 to +70°C. It is inferred from these results that any temperature dependence of the flexural and torsional rigidities of DNA must be rather weak. A less firm inference is that these rigidities actually increase slightly with temperature, possibly in proportion to T, which is weak T dependence in this context. These findings eliminate the possibility that spontaneous transient opening of the DNA structure has any significant effect on the flexural and torsional rigidities of the DNA filament. A review of the most pertinent available data from other experiments concerning spontaneous transient opening of the DNA is presented. The formaldehyde kinetics data do not unequivocally implicate an open base-pair intermediate and provide only an upper limit to the fraction of open base pairs. An alternative nonopening model with a protonated doorway state is proposed to accommodate the hydrogen-exchange data. It is concluded that there is presently no incontrovertible evidence for a fraction of unstacked open base pairs greater than about 10^?4.     

Dynamic light scattering from thin rigid rods: Anisotropy of translational diffusion of tobacco mosaic virus

An Exact theoretical expression for the apparent diffusion coefficient D_app(K) of a thin rigid rod with arbitrary anisotropy of its translational diffusion diffusion coefficient is derived from the first cumulant of its dynamic structure factor. D_app(K) is predicted to reach a limiting plateau value at extermely large values of KL, where K is the scattering vector and L the rod length. Howerver, that limiting plateau value is approached only very slowly along a quasi-plateau with a very gradual slope. Dynamic light-scattering studies have been performed on tobacco mosaic virus from K² = (0.4–20) × 10¹⁰ cm^?2 using 632-8-nm laser radiation. The present data yield D₀ = (4.19 ± 0.10) × 10^?8 cm²/s (corrected to 20,w conditions) and, with literature data to establish L = 2980 Å and the rotational diffusion coefficient D_R = 318s^?1, yield also Δ ≡ D_∥ ? D_⊥ = (1.79 ± 0.38) × 10^?8 cm²/s. The experimental data closely follow the curve of D_app(K) vs K² calcuated for these parameters. The present value of D₀ substantially exceeds all previous dynamic light-scattering values, but is in good aggreement with previous sedimentation data, which were confirmed for the presemt sample. The anisotropy ratio Δ/D₀ = 0.43 ± 0.09 is in accord with theoretical predictions based on the modified Kirkwood algorithm, despite the fact the D₀ lies significantly below its corresponding theoretical value. The present data largely predlude the possibility that both D₀ and Δ/D₀ could simultaneously match their theoretical predictions. We present a detailed comparison of the experimental data with the calculations of Tirado and Garcia de la Torre based on the modified Kirkwood algorithm and with the Broersma formulas.     

The titratable joint phenomenon in ΦW-14 DNA

Dynamic light scattering (DLS) studies are carried out on native ΦW-14 DNA, which has putrescines covalently attached at the methyl groups of half its thymines, and on a chemically modified form of the same DNA in which the ammonium groups of its putrescines are almost completely acetylated. From neutrality to pH 9.6, both forms of ΦW-14 DNA exhibit the same curve of D_app vs K² over the range K² = 0.5 × 10¹⁰ to K² = 20 × 10¹⁰ cm^?2, and this coincides with curves that we have observed for other DNAs. (D_app, apparent diffusion coefficient; K, scattering vector). However, when the pH is raised to pH 10.0–10.2, native ΦW-14 exhibits a spectacular decrease in D_app at large wave vector, whereas the acetylated form shows no sign of such behavior. It is inferred that bound ammonium groups make an essential contribution to the stabilization of titratable joints. Comparing the pH profiles of the absorbance (A₂₆₀) for these two DNAs gives some evidence that base unstacking may be involved in titratable joint formation.     

The concentration of free Ca(2+) in the sarcoplasmic reticulum of frog cut twitch skeletal muscle fibers estimated with tetramethylmurexide

One aim of this article was to determine the resting concentration of free Ca²⁺ in the sarcoplasmic reticulum (SR) of frog cut skeletal muscle fibers ([Ca²⁺]_SR,R) using the calcium absorbance indicator dye tetramethylmurexide (TMX). Another was to determine the ratio of [Ca²⁺]_SR,R to TMX's apparent dissociation constant for Ca²⁺ (K_app) in order to establish the capability of monitoring [Ca²⁺]_SR(t) during SR Ca²⁺ release – a signal needed to determine the Ca²⁺ permeability of the SR. To reveal the properties of TMX in the SR, the surface membrane was rapidly permeabilized with saponin to rapidly dissipate myoplasmic TMX. Results indicated that the concentration of Ca-free TMX in the SR was 2.8-fold greater than that in the myoplasm apparently due to binding of TMX to sites in the SR. Taking into account that such binding might influence K_app as well as a dependence of K_app on TMX concentration, the results indicate an average [Ca²⁺]_SR,R ranging from 0.43 to 1.70 mM. The ratio [Ca²⁺]_SR,R/K_app averaged 0.256, a relatively low value which should not depend on factors influencing K_app. As a result, the time course of [Ca²⁺]_SR(t) in response to electrical stimulation is well determined by, and approximately linearly related to, the active TMX absorbance signal.     

Dynamic light scattering studies of internal motions in DNA. II. Clean viral DNAs

Internal Brownian motions of clean ?29 and λ-DNAs have been studied using photon-correlation techniques at both visible (λ₀ = 632.8 nm) and uv (λ₀ = 363.8 nm) wavelengths. The present dynamic light scattering data, which extend to K² = 19 × 10¹⁰ cm^?2, can in every case be satisfactorily simulated by a Rouse-Zimm model polymer with an appropriate choice of the three model parameters. The effects of pH, salt concentration, single-strand breaks, and molecular weight on those model parameters have also been investigated. Intact clean DNAs exhibit surprisingly little variation with pH from 7.85 to 10.25, with salt concentration from 0.01 NaCl to 5.4M NH₄Cl, or with molecular weight or GC content. The single-strand breaks have no effect at pH 9.46, but produce dramatic changes in the model parameters at pH 10.0 and 10.25, indicating the introduction of titratable joints at those pHs. The failure of either single-strand breaks or a large change in GC content to alter the model parameters in the neutral pH range is a strong indication that local denaturation is not required for those flexions and torsions that dominate the relaxation of fluctuations in the scattered light. The Langevin relaxation time for the slowest internal mode of a particular Rouse-Zimm model derived from the dynamic light scattering data is compared with pertinent literature data extrapolated to the same molecular weight. The present algorithm for determining model parameters from the light-scattering D_app vs K² curve actually yields a Langevin time in fairly good agreement with the literature value. For unknown reasons the light-scattering D₀ values generally exceed those obtained from the molecular weight and sedimentation coefficient by about 20%.     

Cytogenetic Analysis of Hybrids Resistant to Yellow Rust and Powdery Mildew Obtained by Crossing Common Wheat (Triticum aestivum L., AABBDD) with Wheats of the Timopheevi Group (AtAtGG)

The karyotypes of 47 hybrid lines obtained from crosses of common wheat Triticum aestivum L. (cv. Rodina and line 353) with Triticum timopheevii(Zhuk.) Zhuk. (A ^t A ^t GG) and related species T. militinae Zhuk. et Migusch. (A ^t A ^t GG) and T. kiharae Dorof. et Migusch. (A ^t A ^t GGD ^sq D ^sq) were analyzed by C-banding. Most lines were resistant to yellow rust and powdery mildew. The introgression of alien genetic material to the common wheat genome was realized via substitutions of complete A ⁺-,G-, and D-genome chromosomes, chromosome arms, or their fragments. The pattern of chromosome substitutions in resistant lines differed from that in introgressive hybrids selected for other traits. Substitutions of chromosomes 6G, 2A^t, 2G, and 5G were revealed in 31, 23, 18, and 13 lines, respectively. Substitutions of chromosomes 4G, 4A^t, and 6A^t were not observed. In 15 lines, a 5BS.5BL-5GL translocation was identified. High frequency of substitutions of chromosomes 2A^t, 2G, 5G, and 6G indicate that they may carry the resistance genes and that they are closely related to the respective homoeologous chromosomes of common wheat that determines their high compensation ability.     

Speciation of phytate ion in aqueous solution. Non covalent interactions with biogenic polyamines

Abstract

The noncovalent interactions of phytate (Phy^6-) with biogenic amines were studied potentiometrically in aqueous solution, at t= 25°C. Several species are formed in the different H+-Phy^6--amine (A) systems, which have the general formula A_p(Phy)H_q^(12-q)-, with p ≤ 3 and 6 ≤ q ≤ 10. The stability of these species is strictly dependent on the charges involved in the formation equilibria. For the equilibrium pH_iAⁱ⁺ + H_j(Phy)⁽¹²-^j)- = A_p(Phy)H_q(^12q)-, (q = pi + j)we found the relationship logK= aζ (ζ is the charge product of reactants), where a= 0.35(0.03, valid for all the amines; this roughly indicates an average free energy contribution per bond -ΔG⁰ = 4.0 ± 0.2 kJ mol^-1. A slightly more sophisticated equation is also proposed for predicting the stability of these species. Owing to the quite high (partially protonated) phytate charge, the stability of A_p(Phy)H_q^(12-q)- species is quite high, making phytate a strong amine sequestering agent in a wide pH range.     

Three Types of Membrane Excitations in the Marine Diatom Coscinodiscus wailesii

Three types of electrical excitation have been investigated in the marine diatom Coscinodiscus wailesii. I: Depolarization-triggered, transient Cl⁻ conductance, G _Cl (t), followed by a transient, voltage-gated K⁺ conductance, G _K , with an active state a and two inactive states i ₁ and i ₂ in series (a-i ₁-i ₂). II: Similar G _Cl (t) as in Type-I but triggered by hyperpolarization; a subsequent increase of G _K in this type is indicated but not analyzed in detail. III: Hyperpolarization-induced transient of a voltage-gated activity of an electrogenic pump (i ₂-a-i ₂), followed by G _Cl (t) as in Type-II excitations. Type-III with pump gating is novel as such. G _Cl (t) in all types seems to reflect the mechanism of InsP⁻ ₃ and Ca²⁺-mediated G _Cl (t) in the action potential in Chara (Biskup et al., 1999). The nonlinear current-voltage-time relationships of Type-I and Type-III excitations have been recorded under voltage-clamp using single saw-tooth command voltages (voltage range: −200 to +50 mV, typical slope: ±1 Vs⁻¹). Fits of the corresponding models to the experimental data provided numerical values of the model parameters. The statistical significance of these solutions is investigated. We suggest that the original function of electrical excitability of biological membranes is related to osmoregulation which has persisted through evolution in plants, whereas the familiar and osmotically neutral action potentials in animals have evolved later towards the novel function of rapid transmission of information over long distances. Received: 2 December 1999/Revised: 3 March 2000     

Production of precursors for anti-Alzheimer drugs: Asymmetric bioreduction in a packed-bed bioreactor using immobilized D. carota cells

(S)-1-Phenylethanol derivatives, which are the precursors of many pharmacological products, have also been used as anti-Alzheimer drugs. Bioreduction experiments were performed in a batch and packed-bed bioreactor. Then, the kinetics constants were determined by examining the reaction kinetics in the batch system with free and immobilized carrot cells. Also, the effective diffusion coefficient (D_e) of acetophenone in calcium alginate-immobilized carrot cells was investigated. Kinetics constants for free cells, which are intrinsic values, are reaction rate V_max?=?0.052?mmol?L^?1?min^?1, and constants of the Michaelis–Menten K_M?=?2.31?mmol?L^?1. Kinetics constants for immobilized cells, which are considered apparent values, are V_{max, app}?=?0.0407?mmol?L^?1 min^?1, K_{M, app}?=?3.0472?mmol?L^?1 for 2?mm bead diameter, and V_{max, app}?=?0.0453?mmol?L^?1 min^?1, K_{M, app}?=?4.9383?mmol?L^?1 for 3?mm bead diameter. Average value of effective diffusion coefficient of acetophenone in immobilized beads was determined as 1.97?×?10^?6?cm²?s^?1. Using immobilized carrot cells in an up-flow packed-bed reactor, continuous production of (S)-1-phenylethanol through asymmetric bioreduction of acetophenone was performed. The effects of the residence time and concentrations of substrate were investigated at pH 7.6 and 33°C. Enantiomerically pure (S)-1-phenylethanol (ee?>?99%) was produced with 75% conversion at 4-hr residence time.     

Dynamic light-scattering studies on thermal motions of native DNAs in solution

The basic character of dynamic light-scattering properties of native DNA was investigated on two DNA samples. The degree of non-single exponentiality of photocount correlation functions. C(t), and its dependence on Kquantitatively characterized by two methods. The spectral linewidth. Γ₃, determined from C(t) exhibits a K dependence near to but significantly different from the prediction for Rouse-Zimm (RZ) chains by Dubois-Violelte and De Gennes: It is inferred from data on λ-DNA that the exponent in She K dependence of the spectral linewidth for native DNA takes a value larger than 3 in the K region corresponding to the ‘K³’ region for RZ chains. These results are in good agreement with the prediction from the dynamic theory of semiflexible chains presented by one of us (K.S.). The apparent diffusion coefficients are fairly insensitive to DNA concentration and ionic strength at large K. On the other hand, it is indicated that the stiffness of native DNA may vary with temperature even in a temperature range substantially lower than thai of melting.     

Activity coefficients of intracellular Na+ and K+ during development of frog oocytes

Summary The chemical activities, (a), of Na⁺ and K⁺ were determined in large mature and in small immature frog oocytes, using open-tipped micropipettes and ionselective microelectrodes. The average chemical concentrations,c, of Na⁺ and K⁺ were determined by spectrophotometry and by electron probe X-ray microanalysis. The apparent activity coefficient (^app) was calculated for each ion as the ratio,a/c.With development, (a _Na/a _K) decreased four to fivefold and (c _Na/c _K) increased six to sevenfold. In the large mature oocytes, _Na ^app was measured to be 0.08±0.02 and _K ^app lay within the range 1.15±0.03 to 1.29±0.04, constituting the smallest value for Na⁺ and largest value for K⁺, respectively, thus far reported. This intracellular value of _K ^app was substantially greater than the activity coefficient of K⁺ in the external medium (0.76). The data suggest that the inequality of _Na ^app and _K ^app in this and probably other cells reflects the development of subcellular compartmentalization of ions. Possible intracellular sites of ionic compartmentalization are considered.     

Blood-Brain Barrier Permeation: Molecular Parameters Governing Passive Diffusion

53 compounds with clinically established ability to cross or not to cross the blood-brain barrier by passive diffusion were characterized by means of surface activity measurements in terms of three parameters, i.e., the air-water partition coefficient, K _aw , the critical micelle concentration, CMC _D , and the cross-sectional area, A _D . A three-dimensional plot in which the surface area, A _D , is plotted as a function of K ⁻¹ _aw and CMC _D shows essentially three groups of compounds: (i) very hydrophobic compounds with large air-water partition coefficients and large cross-sectional areas, A _D > 80 ?² which do not cross the blood-brain barrier, (ii) compounds with lower air-water partition coefficients and an average cross-sectional area, A _D ≅ 50 ?² which easily cross the blood-brain barrier, and (iii) hydrophilic compounds with low air-water partition coefficients (A _D < 50 ?²) which cross the blood-brain barrier only if applied at high concentrations. It was shown that the lipid membrane-water partition coefficient, K _lw , measured previously, can be correlated with the air-water partition coefficient if the additional work against the internal lateral bilayer pressure, π _bi = 34 ± 4 mN/m is taken into account. The partitioning into anisotropic lipid membranes decreases exponentially with increasing cross-sectional areas, A _D , according to K _lw =const. K _aw exp(−A _D π _bi /kT) where kT is the thermal energy. The cross-sectional area of the molecule oriented at a hydrophilic-hydrophobic interface is thus the main determinant for membrane permeation provided the molecule is surface active and has a pK _a > 4 for acids and a pK _a < 10 for bases. Received: 7 April 1998/Revised: 25 June 1998     

Photoaptameric heterodimeric constructs as a new approach to enhance the efficiency of formation of photocrosslinks with a target protein

Using DNA aptamers selectively recognizing anion-binding exosites 1 and 2 of thrombin as a model, it has been demonstrated that their conjugation by a poly-(dT)-linker (ranging from 5 to 65 nucleotides (nt) in length) to produce aptamer heterodimeric constructs results into affinity enhancement. At the linker lengths ranged from 35 to 55 nt the apparent dissociation constants (K _D^app) measured using the optical biosensor Biacore-3000 for complexes of thrombin with the heterodimeric constructs reached minimum values (K _D^app) = 0.2–0.4 nM), which were approximately 30-fold less than for the complexes with the initial aptamers. A photoaptamer heterodimeric construct was designed connecting photoaptamer and aptamer sequences with the poly-(dT)-linker of 35 nt long. The photoaptamer used could form photo-induced cross-links with the exosite 2 of thrombin and the aptamer could bind to the exosite 1. The (K _D^app value for the photoaptamer construct was approximately 40-fold less than that for the primary photoaptamer (5.3 and 190 nM, respectively). Upon exposure of the equimolar mixtures of thrombin with the photoaptamer construct to the UV radiation at 308 nm the equal yield of the crosslinked complexes was observed at concentrations, which were lower by two orders of magnitude than in the case of the primary photoaptamer. It was found that concurrently with crosslinking to thrombin a photo-induced inactivation of the photoaptamer occurs presumably due to formation of the intermolecular crosslinking.     

Size adaptation of turing prepatterns

Spontaneous pattern formation may arise in biological systems as primary and secondary bifurcations to nonlinear parabolic partial differential equations describing chemical reaction-diffusion systems. Such Turing prepatterns have a specified geometry as long as D/R ² (the diffusion coefficient of the morphogen D divided by the square of a characteristic length) is confined to a (usually) limited interval. As real biochemical systems like cleaving eggs or early embryos vary considerably in size, Turing prepatterns are unable to maintain a specified prepattern-geometry, unless D/R ² is varied as well. We show, that actual biochemical control systems may vary D _app/R², where D _app(k) is an apparent diffusion constant, dependent on enzyme regulated rate constants, and that such simple control systems allow Turing structures to adapt to size variations of at least a factor 10³ (linearly), not only in large connected cell systems, but in single cells as well.     

A model of photosystem II for the analysis of fast fluorescence rise in plant leaves

The polyphasic patterns of fluorescence induction rise in pea leaves in vivo and after the treatment with ionophores have been studied using a Plant Efficiency Analyzer. To analyze in detail photosystem II (PS II) electron transfer processes, an extended PS II model was applied, which included the sums of exponential functions to specify explicitly the light-driven formation of the transmembrane electric potential (ΔΨ(t)) as well as pH in the lumen (pH_L(t)) and stroma (pH_S(t)). PS II model parameters and numerical coefficients in ΔΨ(t), pH_L(t), and pH_S(t) were evaluated to fit fluorescence induction data for different experimental conditions: leaf in vivo or after ionophore treatment at low or high light intensity. The model imitated changes in the pattern of fluorescence induction rise due to the elimination of transmembrane potential in the presence of ionophores, when ΔΨ = 0 and pH_L(t), pH_S(t) changed to small extent relative to control values in vivo, with maximum ΔΨ(t) ∼ 90 mV and ΔΨ(t) ∼ 40 mV for the stationary state at ΔpH ≅ 1.8. As the light intensity was increased from 300 to 1200 μmol m⁻² s⁻¹, the heat dissipation rate constants increased threefold for nonradiative recombination of P680⁺Phe⁻ and by ∼30% for P680⁺Q_A⁻. The parameters ΔΨ, pH_S and pH_L were analyzed as factors of PS II redox state populations and fluorescence yield. The kinetic mechanism of fluorescence quenching is discussed, which is related with light-induced lumen acidification, when ⁺Q_A⁻ and P680⁺ recombination probability increases to regulate the Q_A reduction.     

Variable spread of X inactivation affecting the expression of different epitopes of the Hya gene product in mouse B-cell clones

Cloned B-cell lines from a female T16H/XSxr mouse in which Tdy expression was suppressed due to X inactivation and from a male X/XSxr mouse, both of the (kxb)F₁ haplotype, were examined for H-Y expression. This was determined both by their ability to act as targets for H-2^k and H-2^b-restricted H-Y-specific cytotoxic T cells and by their ability to stimulate the proliferation of H-2K^k, H-2D^b (class I) and A^b (class II)-restricted T-cell clones. In B-cell clones from the T16H/XSxr mouse, expression of H-Y/D^b exhibited partial X inactivation and only a proportion ( 30%) of the cells were targets for or stimulated H-2D^b-restricted H-Y-specific T cells. In contrast, H-Y eiptopes restricted by H-2^k (H-Y/K^k, H-Y/D^k) and A^b (H-Y/A^b) exhibited no X inactivation. Furthermore, no inactivation of H-Y/D^b, H-Y/A^b, or H-Y^k was observed in the male X/XSxr mouse. These results indicate that the T16H/XSxr female is a mosaic, as a result of the variable spread of X inactivation into the Sxr region. They further suggest that the H-Y antigen recognized in association with H-2^k and H-2D^b class I molecules and A^b class II molecules may be the product of more than one gene.     

The effect of body size on food consumption, absorption efficiency, respiration, and ammonia excretion by the inland silverside, Menidia beryllina (Cope) (Osteichthyes: Atherinidae)

The inland silverside, Menidia beryllina (Cope), is an annual zooplanktivore that occurs in estuarine and freshwater habitats along the Atlantic and Gulf of Mexico coasts and drainages of the United States. Experiments were conducted at 25 ± 1°C to quantify the relationship between mean dry weight (W_D) and rates of energy gain from food consumption (C), and energy losses as a result of respiration (R) and ammonia excretion (E) during routine activity and feeding by groups of fish. The absorption efficiency of ingested food energy (A) was also quantified. Rates of C, E, and R increased with W_D by factors (b in the equation y = aW_D^b) equal to 0.462, 0.667, and 0.784, respectively. Mean (±SE) rates of energy loss during feeding were 1.6 ± 0.1 (R) and 3.4 ± 0.6 (E) times greater than those for unfed fish. Absorption efficiency was independent of W_D and estimated to be 89% of C. From these measurements, the surplus energy available for growth and activity (G) and growth efficiency (K₁) were estimated. Over the range in sizes of juveniles and adults (5–500 mg W_D), predicted G and K₁ values decreased from 7.42 to 0.20 J mg fish^?1 day^?1 and 63 to 21%, respectively. Measured and predicted bioenergetic parameters are discussed within an ecological context for a northern population of this species.     

Effect of temperature on DNA secondary structure in the absence and presence of 0.5 M tetramethylammonium chloride

Changes in the average secondary structures of three different linear DNAs over the premelting region from 5 to 60°C were investigated by measuring their CD spectra and also their torsion elastic constants (〈α〉) by time-resolved fluorescence polarization anisotropy. For one of these DNAs, the HaeII fragment of pBR322, the apparent diffusion coefficients [D_app (k)] at small and large scattering vectors (k) were also measured by dynamic light scattering. With increasing temperature, all three DNAs exhibited typical premelting changes in their CD spectra, and these were accompanied by 1.4- to 1.7-fold decreases in 〈α〉. Also for the 1876 base pair fragment, D_app(k) at large scattering vectors, which is sensitive to the dynamic bending rigidity, decreased by 17%, even though there was no change at small scattering vectors, where D_app(k) = D₀ is the translational diffusion coefficient of the center-of-mass. These observations demonstrate conclusively that the premelting CD changes of these DNAs are associated with a significant change in average secondary structure and mechanical properties, though not in persistence length. In the presence of 0.5 M tetramethylammonium chloride (TMA-Cl) the premelting change in CD is largely suppressed, and the corresponding changes in 〈α〉 and D_app(k) at large scattering vectors are substantially diminished. These observations suggest that TMA-Cl, which binds preferentially to A · T-rich regions and stabilizes those regions (relative to G · C-rich regions) against melting, effectively stabilizes the prevailing low-temperature secondary structure sufficiently that the DNA is effectively trapped in that state over the temperature range observed. © 1998 John Wiley & Sons, Inc. Biopoly 45: 503–515, 1998     

Characterization and Localization of Adenosine A2 Receptors in Bovine Rod Outer Segments

Abstract: Previous work from this laboratory has shown that retinal adenosine A₂ binding sites are localized over outer and inner segments of photoreceptors in rabbit and mouse retinal sections. In the present study, adenosine receptor binding has been characterized and localized in membranes from bovine rod outer segments (ROS). Saturation studies with varying concentrations (10–150 nM) of 5′-(N-[2,8-³H]ethylcarboxamido)adenosine ([³H]NECA) and 100 μg of ROS membrane protein show a single site with a K_D of 103 nM and a B_max of 1.3 pM/mg of protein. Cold Scatchards, which used nonradiolabeled NECA (concentrations ranging from 10 nM to 250 nM) in competition with a fixed amount of [³H]NECA (30 nM), demonstrated the presence of a low-affinity site (K_D, 50 μM) in addition to the high-affinity site. To confirm the presence of A_2abinding sites, saturation analyses with 2-p-(2-[³H]-carboxyethyl)phenylamino-5′-N-ethylcarboxamido adenosine (0–80 nM) also revealed a single population of high-affinity A_2a receptors (K_D, 9.4 nM). The binding sites labeled by [³H]NECA appear to be A₂ receptor sites because binding was displaced by increasing concentrations of 5′-(N-methylcarboxamido)adenosine and 2-chloroadenosine. ROS were fractionated into plasma and disk membranes for localization studies. Receptor binding assays, used to determine specific binding, showed that the greatest concentration of A₂ receptors was on the plasma membranes. Therefore, adenosine A₂ receptors are in a position to respond to changes in the concentration of extracellular adenosine, which may exhibit a circadian rhythm.