Die Lichtempfindlichkeit der Circadianen rhythmik dreier Höhlenkäfer-arten der Gattung Laemostenus

Laemostenus terricola and oblongus are troglophile (facultative) cavernicolous animals. Laemostenus navarricus is troglobite; outside of caves and cave entrances it cannot survive. In the three species, the period of spontaneous locomotor activity is longest under LL conditions of 10^?4 and 10^?2 lux. In both. DD and more intense LL. the period becomes shorter. The precision of the animals activity periods was computed by periodogram analysis. In DD and LL of 10^?4 lux the rhythm of locomotor activity of L. navarricus is extinct or very unprecise, in LL of 10^?2 and 10⁰ lux the periods are much more exact. L. oblongus is always periodically active under DD and weak LL conditions; like L. navarricus its activity periods are more precise in brighter LL. On the other hand the activity periods of L. terricola are relatively precise in DD and LL of 10^?4 lux and become more unprecise in increasing light intensity. More than 50% of the tested animals were synchronised by LDs of $\text{0}\text{10}{}^{\mathrm{?4}}$. $\text{0}\text{10}{}^{\mathrm{?2}}$, and $\text{0}\text{10}{}^0$ lux.Because of the extreme light sensitivity of their circadian system, the three species are adapted to twilight habitats (cave entrances). Because of its aperiodicity under DD conditions L. navarricus is additionally adapted to a life in constant darkness (inside of caves). For this and other physiological and morphological reasons the troglobite L. navarricus represents a relatively early stage of regressive evolution under cave conditions.     

Kontrollmechanismen der Glycolyse

Control mechanisms of enzymic reactions are generally based on interactions between activators, inhibitors, substrates and products with enzyme proteins or on induction and repression of enzyme synthesis. All main types of control can be recognized in glycolysis. They are the basis of the network which controls the over-all glycolytic function and operates according to the feed-back principle. — Enzyme profiles may not be used for a functional definition of the metabolic state. Enzyme activities are governed by a variety of control mechanisms, which can best be recognized by steady-state and transient state analysis of metabolites and by an analysis of the system's response in titration experiments with pure enzymes under conditions whereby the system displays oscillatory behaviour of its over-all flux. The important parameter for the definition of the metabolic state is the net-flux through the system, since this parameter, along with the steady-state levels of the meabolites, gives the steady-state flux pattern, reveals the kinetic state of enzymic reactions and points to control points of metabolism. Continuous glycolytic oscillations in a cell-free extract ofSaccharomyces carlsbergensis have been observed over a period of 22 hours with a constant frequency of 0.17 min^?1 and a rate of 7.2 nMol ethanol per mg protein per min. Titration of such an extract by pure yeast enzymes reveals the gain (FDP, ADP) and damping components (ATP), which are fed back to the enzymes PFK and PK, respectively, PFK, PGK and PK operating as the control units. On the basis of the titration data as well as metabolite and enzyme activity phase relationship, the mechanism of this oscillation can be understood as a crossed feed-back interaction. Furthermore, it is discussed as the biochemical model of a physiological clock mechanism.     

Light activates output from evening neurons and inhibits output from morning neurons in the Drosophila circadian clock

Animal circadian clocks are based on multiple oscillators whose interactions allow the daily control of complex behaviors. The Drosophila brain contains a circadian clock that controls rest–activity rhythms and relies upon different groups of PERIOD (PER)–expressing neurons. Two distinct oscillators have been functionally characterized under light-dark cycles. Lateral neurons (LNs) that express the pigment-dispersing factor (PDF) drive morning activity, whereas PDF-negative LNs are required for the evening activity. In constant darkness, several lines of evidence indicate that the LN morning oscillator (LN-MO) drives the activity rhythms, whereas the LN evening oscillator (LN-EO) does not. Since mutants devoid of functional CRYPTOCHROME (CRY), as opposed to wild-type flies, are rhythmic in constant light, we analyzed transgenic flies expressing PER or CRY in the LN-MO or LN-EO. We show that, under constant light conditions and reduced CRY function, the LN evening oscillator drives robust activity rhythms, whereas the LN morning oscillator does not. Remarkably, light acts by inhibiting the LN-MO behavioral output and activating the LN-EO behavioral output. Finally, we show that PDF signaling is not required for robust activity rhythms in constant light as opposed to its requirement in constant darkness, further supporting the minor contribution of the morning cells to the behavior in the presence of light. We therefore propose that day–night cycles alternatively activate behavioral outputs of the Drosophila evening and morning lateral neurons.     

Light Activates Output from Evening Neurons and Inhibits Output from Morning Neurons in the Drosophila Circadian Clock

Animal circadian clocks are based on multiple oscillators whose interactions allow the daily control of complex behaviors. The Drosophila brain contains a circadian clock that controls rest–activity rhythms and relies upon different groups of PERIOD (PER)–expressing neurons. Two distinct oscillators have been functionally characterized under light-dark cycles. Lateral neurons (LNs) that express the pigment-dispersing factor (PDF) drive morning activity, whereas PDF-negative LNs are required for the evening activity. In constant darkness, several lines of evidence indicate that the LN morning oscillator (LN-MO) drives the activity rhythms, whereas the LN evening oscillator (LN-EO) does not. Since mutants devoid of functional CRYPTOCHROME (CRY), as opposed to wild-type flies, are rhythmic in constant light, we analyzed transgenic flies expressing PER or CRY in the LN-MO or LN-EO. We show that, under constant light conditions and reduced CRY function, the LN evening oscillator drives robust activity rhythms, whereas the LN morning oscillator does not. Remarkably, light acts by inhibiting the LN-MO behavioral output and activating the LN-EO behavioral output. Finally, we show that PDF signaling is not required for robust activity rhythms in constant light as opposed to its requirement in constant darkness, further supporting the minor contribution of the morning cells to the behavior in the presence of light. We therefore propose that day–night cycles alternatively activate behavioral outputs of the Drosophila evening and morning lateral neurons.     

Zur Determination der zeitlichen Verteilung von Fortpflanzungsprozessen in Laborkulturen des PolychaetenTyposyllis prolifera

Factors controlling the timing of reproduction in laboratory cultures of the polychaete Trposyllis prolifera.Typosyllis prolifera (Krohn) from Pore (Yugoslavia) has been cultured for 12 successive generations. The life cycle of the species in the laboratory is described briefly. During their life individuals reproduce several times (up to 15) by stolonization which, under constant laboratory conditions (LD 16:8, 20 °C), is cyclic and takes place about every 30 days. Based on the investigations of Durchon (1959) and Wissocq (1966), experiments on extirpation and transplantation of the proventriculus have been carried out. The results suggest that an endocrine system anatomically connected to the proventriculus is important in the control of reproduction. Most likely, the endogenous reproductive cycle of an individual is controlled by periodical changes of the activity of this system. During the period following stolonization, the endocrine system of the proventriculus, which at this time shows its maximal activity, inhibits sexual development and enables regeneration of the segments lost as stolon. A subsequent decrease of the hormonal activity induces sexual maturation and epitokous metamorphosis, thus leading to further stolonization. Exogenous factors influencing the timing of reproduction probably affect the endocrine function of the proventriculus. Short-day photoperiods (LD 10:14) and low temperatures (12 °C) given simultaneously (i.e. winter conditions) totally suppress reproduction. Under normally favourable conditions (LD 16:8, 20 °C), reproductive processes can be prevented by starving or amputation of caudal segments. In all these cases, however, stolonization can be induced by removing the proventriculus. Exogenous factors also play a decisive role in synchronizing reproductive events within the species population. Under field conditions reproduction shows a lunar periodicity. The endogenous reproductive cycles of cultured specimens can be synchronized by a periodical light regime simulating the change of the moon.     

Untersuchungen zum circadianen Sekretionszyklus der neurosekretorischen Kerngebiete der Maus*

Abstract Investigations on the circadian cycle of secretion in the neurosecretory nuclei of mouse. The circadian cycle of secretion of both hypothalamic neurosecretory nuclei N. supraopticus and N. paraventricularis in mice was studied by histological, histochemical, biochemical and electron-microscopical methods. Both areas undergo a circadian secretion cycle (24 h-rhytm). The activity of metabolism increases at the beginning of night, whereas by day the cells are considered to be in a phase of restitution and transportation of the secretion. Especially increases the volume of the cell nuclei (synthesis of RNA and proteins) and the activity of the extralysosomal unspecific esterase rises to maximal values. From midnight till the beginning of the light-phase the cells show a maximal volume of the nuclei and an increased secretion as well as higher activity of acid phosphatase and lactate dehydrogenase. Unspecific esterase activity, on the other hand, diminished. There was a noticable increase in the quantity of neurosecretion in both areas, it reached a maximum at the beginning of the light-phase, whereas the volume of the nuclei and the activity of acid phosphatase and thiamine pyrophosphatase diminished. The neurosecretion is more and more transported to the hypophysis. The cells restitute during the light-phase and reach their starting level.     

Zur Konstanz der Aminosäurenzusammensetzung im Eiweißanteil des Tabakmosaikvirus

Summary Different cultural temperatures (greenhouse and constant temperature room at 35° C) or different hosts (non-solanaceous plants) had no influence on the amino acid composition of the proteins of certain tobacco mosaic virus strains (Aach 1960, 1961). The protein analyses have now been completed by estimating the electrophoretic mobility of the different viral preparations. The new data confirm that the characteristics of TMV protein are constant in spite of variations of synthesis conditions and depend only upon the infecting strain.

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Durchgeführt mit einer Sachbeihilfe der Deutschen Forschungsgemeinschaft     

Tagesperiodische Schwankungen der Vorzugshelligkeit bei Vögeln

Zusammenfassung 7 Stare (Sturnus vulgaris), 2 Beos (Gragula religiosa) und 1 Amsel (Turdus merula) wurden einzeln für jeweils mindestens 14 Tage in einer Apparatur gehalten, in der sie zwischen zwei oder drei unterschiedlich hell, aber zeitlich konstant beleuchteten Kästen wählen konnten. In jedem Kasten wurde ihre lokomotorische Aktivität registriert. Acht der zehn Vögel zeigten eine circadiane Periodik der Aktivität und, korreliert da mit, eine circadiane Periodik der bevorzugten Beleuchtungsstärke; die Vorzugshelligkeit nahm bei steigender Gesamtaktivität zu; zum Schlafen suchten die Vögel stets den dunkleren Kasten auf. Diese strenge Korrelation zwischen Aktivitätsmenge und Vorzugshelligkeit galt bei einem Tier nur für bestimmte Teile seiner Aktivitätsperiodik, im übrigen aber auch für solche Vögel, deren Aktivität in kurzperiodischen Schüben geordnet oder aperiodisch war.Es wird auf die ökologische Bedeutung dieser Befunde hingewiesen und die Frage nach möglichen Konsequenzen für die Tierhaltung erörtert.

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Summary 7 Starlings (Sturnus vulgaris), 2 Beos (Gracula religiosa) and 1 Blackbird (Turdus merula) were kept for at least 14 days, each in isolation, in an apparatus in which they were able to chose between 2 or 3 chambers with different, but constant, light intensities. Locomotor activity was measured in each chamber. 8 of 10 birds exhibited, at least temporarily, a circadian rhythm of activity and correllated to it a circadian rhythm in the preferred light intensity: the preferred intensity increased with increasing activity, and the birds always selected the dark chamber for sleeping. This close correlation between activity level and preferred light intensity was absent only in one animal at particular times of his period, and was otherwise typical also for birds with frequent periodic or aperiodic bursts of activity. The ecological significance of these findings is discussed, and possible consequences for the maintenance of animals in captivity are considered.

     

Elektronenmikroskopischer Nachweis der Adenosintriphosphatase in der Synovialmembran der Ratte

Zusammenfassung Nach Inkubation im Medium von Wachstein und Meisel von in Glutaraldehyd fixierten Gewebsblöckchen wurde die Verteilung der ATPase-Aktivität in der Synovialmembran an 20 Kniegelenken von Wistarratten demonstriert. Von allen Geweben des Kniegelenks zeigt die Synovialmembran die stärkste ATPase-Aktivität. Das Reaktionsprodukt (RP) liegt am Plasmalemm der Synovialzellen, in den Zwischenzellräumen und an der Kapillarwand. Die Zellorganellen und die Kernmembran enthalten kein Reaktionsprodukt. Die stärkste Enzymaktivität zeigen die B-Zellen des Synovialgewebes. Das RP befindet sich in der Zellmembran und in pinozytotischen Bläschen. Bei A-Zellen liegt das RP am Plasmalemm, an zytoplasmatischen Ausläufern und an Membranen einiger oberflächlich gelegener Vakuolen. Ähnlich ist die Verteilung des RP bei den tiefer gelegenen Makrophagen. Die Enzymaktivität der Kapillaren ist an der äußeren Membran der Endothelzellen lokalisiert. Zonulae adhaerentes sowie das Diaphragma der Endothelzellen der gefensterten Kapillaren sind frei von RP. — Die ATPase-Aktivität der Synovialmembran wird mit deren sekretorischen Aufgaben in Zusammenhang gebracht.

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Ultrahistochemical localization of adenosine triphosphatase activity in the synovial membrane of rats

Summary After incubation in Wachstein-Meisel medium of glutaraldehyde fixed tissue blocks, the adenosine triphosphatase activity of the synovial membrane from 20 knee joints of Wistar rats was demonstrated. Of all kneejoint tissues the synovial membrane exhibits the strongest ATPase activity. The reaction product (RP) is located along the plasmalemma of the synovial cells, in the intercellular spaces, and over the capillary walls. No reaction product is seen in cellular organelles and the nuclear membrane. B-cells show the strongest enzyme activity. Here RP is found along the cellular membrane and within pinocytotic vesicles. In A-cells RP is deposited along the plasmalemma of the cytoplasmic processes and the membranes of some of the superficially situated vacuoles. A similar distribution of RP is found in deeper situated macrophages. The enzyme activity of capillaries is located over the outer membrane of the endothelial cells. Zonulae adhearentes and the diaphragm of the endothelial pores in fenestrated capillaries, too, do not exhibit reaction product. — From these results the role of active membrane transport for secretory functions in the synovial membrane is discussed.

     

Circadian Rhythms Differ between Sexes and Closely Related Species of Nasonia Wasps

Activity rhythms in 24 h light-dark cycles, constant darkness, and constant light conditions were analyzed in four different Nasonia species for each sex separately. Besides similarities, clear differences are evident among and within Nasonia species as well as between sexes. In all species, activity in a light-dark cycle is concentrated in the photophase, typical for diurnal organisms. Contrary to most diurnal insect species so far studied, Nasonia follows Aschoff''s rule by displaying long (>24 h) internal rhythms in constant darkness but short (<24 h) in constant light. In constant light, N. vitripennis males display robust circadian activity rhythms, whereas females are usually arrhythmic. In contrast to other Nasonia species, N. longicornis males display anticipatory activity, i.e. activity shortly before light-on in a light-dark cycle. As expected, N. oneida shows activity patterns similar to those of N. giraulti but with important differences in key circadian parameters. Differences in circadian activity patterns and parameters between species may reflect synchronization of specific life-history traits to environmental conditions. Scheduling mating or dispersion to a specific time of the day could be a strategy to avoid interspecific hybridization in Nasonia species that live in sympatry.     

The effect of restricted feeding on the wheel-running activity rhythms of the predatory marsupial Dasyurus viverrinus

Summary The effects of restricted feeding schedules on the circadian rhythms of wheel-running of Dasyurus viverrinus were examined under a light/dark cycle and in constant darkness (experiment 1) and in constant light (experiment 2). The results of the 2 experiments showed that: (1) in contrast to the light/dark cycle, restricted feeding is only a weak zeitgeber for the wheel-running activity rhythms of D. viverrinus; (2) restricted feeding elicits meal anticipatory activity in D. viverrinus comparable to that elicited by restricted feeding in the rat; (3) transient cycles of the anticipatory activity free-run with a period different to that of the main component of activity for several cycles after the termination of restricted feeding; and (4) activity suggestive of beating between 2 oscillators occurs during restricted feeding and after the termination of restricted feeding. Taken together the latter 3 observations suggest that the activity rhythms of D. viverrinus are controlled by at least 2 separate circadian oscillators.     

Hohe Strahlenresistenz der Aktivität und der lichtinduzierten Aktivitätssteigerung der NADP-abhängigen Glycerinaldehyd-3-Phosphat-Dehydrogenase in Ankistrodesmus braunii

Summary In Ankistrodesmus braunii, a unicellular alga, the activity of NADP-dependent glyceraldehyde phosphate dehydrogenase (GPD) in darkness is very resistant to X-rays. The light-induced increase in the activity of this enzyme within one minute is also not affected by irradiation. Doses up to 1060 Krad had no inhibitory effect. The increase of GPD activity during the life cycle, however, is very sensitive to X-rays.     

Endogenous Rhythms in Photosynthesis, Sucrose Phosphate Synthase Activity, and Stomatal Resistance in Leaves of Soybean (Glycine max [L.] Merr.)

Experiments were conducted with soybean (Glycine max [L.] Merr. cv `Ransom') plants to determine if diurnal rhythms in net carbon dioxide exchange rate (CER), stomatal resistance, and sucrose-phosphate synthase (SPS) activity persisted in constant environmental conditions (constant light, LL; constant dark DD) and to assess the importance of these rhythms to the production of nonstructural carbohydrates (starch, sucrose, and hexose). Rhythms in CER, stomatal resistance, and SPS activity were observed in constant environmental conditions but the rhythms differed in period length, amplitude, and phase. The results indicated that these photosynthetic parameters are not controlled in a coordinated manner. The activity of UDPG pyrophosphorylase, another enzyme involved in sucrose formation, did not fluctuate rhythmically in constant conditions but increased with time in plants in LL. In LL, the rhythm in CER was correlated positively with fluctuations in total chlorophyll (r = 0.810) and chlorophyll a (r = 0.791) concentrations which suggested that changes in pigment concentration were associated with, but not necessarily the underlying mechanism of, the rhythm in photosynthetic rate. Assimilate export rate, net starch accumulation rate, and leaf sucrose concentration also fluctuated in constant light. No single photosynthetic parameter was closely correlated with fluctuations in assimilate export during LL; thus, assimilate export may have been controlled by interactions among the endogenous rhythms in CER, SPS activity, or other metabolic factors which were not measured in the present study.     

Glycolytic enzyme activity in developing red and white muscle

The activities of the constant proportion enzymes of the Embden-Meyerhof chain (triose phosphate isomerase (TIM), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoglycerate kinase (PGK), phosphoglycerate mutase (PGM) and enolase (ENOL)), and the activity of lactic dehydrogenase (LDH) were studied in developing red (trapezius) and white (longissimus) muscles of the pig from a fetal stage to 24 weeks postnatal. Both muscles were differentiated by two weeks postnatal in the sense that they had reached the adult level of enzyme activity. Enzyme activities were two- to three-fold greater in the longissimus than in the trapezius. Enzyme activity ratios based on GAPDH were not consistent in the fetal and day 1 samples but were consistent during later stages of growth. Ratios of enzyme activity based on activity at 105 days gestation revealed that TIM, PGK and PGM are grouped and follow the same pattern, but GAPDH and ENOL are quite different from each other and from the pattern shown by TIM, PGK and PGM. The constant proportion concept in developing muscle is therefore questioned.     

Zum Mechanismus der biologischen 24-Stunden-Periodik

Summary Using the physical and mathematical basis given in two foregoing papers, a differential equation is proposed for a model of the biological 24-hour-periodicity. This oscillation equation contains two characteristic non-linearities describing the self-sustaining property and the circadian rule. The right side of the equation (external force) represents the controlling environmental conditions, mainly the intensity of illumination. Solutions were obtained for different environmental conditions using a digital computer.Under constant conditions the solution of the equation describes oscillations self-sustained within a certain range of environmental conditions. In this range the oscillations fulfil the circadian rule, e.g. for light-active organisms: The frequency and the mean value of the oscillation increase with increasing light intensity; with an additional (arbitrary) threshold separating activity time and rest time for describing an activity rhythm, the (activity time rest time) ratio and the total amount of activity also increase.Under periodically changing environmental conditions five properties of the Zeitgeber used (two distinct intensities with twilight transitions) are variable and varied: The range of oscillation of the Zeitgeber, its frequency, its mean value, its L D ratio (time relation of light time and dark time), and the duration of the twilights. The most important of the examined properties was the phase angle difference between the (forced) oscillation and the (forcing) Zeitgeber. The general result for light-active organisms was: The phase of the oscillation advances relative to the Zeitgeber (in sofar as the oscillation is synchronized) if the period of the Zeitgeber, or its mean value, or its LD ratio, or the duration of the twilights increase. In dark-active organisms, the relation between phase angle difference and the mean value or the LD ratio is reversed. Exceptions to this general rule exist in the relation between phase angle difference and L D ratio if the free running period of the oscillation deviates too much from the period of a weak Zeitgeber (mainly in dark-active organisms) or if the duration of the twilights is too short (especially if the transitions are rectangular).Single exposures to light (or darkness) during constant conditions result in phase shifts depending in direction and amount on the phase of the oscillation at which the disturbance occured. The resulting response curves depend in range and form on the one hand on the time of measuring the phase shifts (either immediately or after several periods — in the steady state — following the disturbance) and, on the other hand, on the intensity of the initial illumination, on the duration, and on the intensity of the exposures, each in a different manner. Moreover, response curves effective in LD conditions deviate from those measured under constant conditions; the reason being the difference in the energy state of the oscillations in the two conditions. Therefore, it is impossible to derive the phase angle difference between the oscillation and a Zeitgeber in self-sustained oscillations from the measurement of response curves alone.The oscillation equation used contains only one free parameter, the frequency coefficient. If this coefficient is changed, the equation describes other biological rhythms. For instance, with a high value it describes the behaviour of single nerve cells, and that not only in cases of spontaneous rhythmicity (e.g. receptor cells) but also in cases of reactions to single or rhythmic stimuli. Moreover, the derived characteristics of the equation — especially the non-linearities — seem to be significant for other biological problems such as control mechanisms.     

Nucleosome Spacing Generated by ISWI and CHD1 Remodelers Is Constant Regardless of Nucleosome Density

Arrays of regularly spaced nucleosomes are a hallmark of chromatin, but it remains unclear how they are generated. Recent genome-wide studies, in vitro and in vivo, showed constant nucleosome spacing even if the histone concentration was experimentally reduced. This counters the long-held assumption that nucleosome density determines spacing and calls for factors keeping spacing constant regardless of nucleosome density. We call this a clamping activity. Here, we show in a purified system that ISWI- and CHD1-type nucleosome remodelers have a clamping activity such that they not only generate regularly spaced nucleosome arrays but also generate constant spacing regardless of nucleosome density. This points to a functionally attractive nucleosome interaction that could be mediated either directly by nucleosome-nucleosome contacts or indirectly through the remodelers. Mutant Drosophila melanogaster ISWI without the HAND-SANT-SLIDE (HSS) domain had no detectable spacing activity even though it is known to remodel and slide nucleosomes. This suggests that the role of ISWI remodelers in generating constant spacing is not just to mediate nucleosome sliding; they actively contribute to the attractive interaction. Additional factors are necessary to set physiological spacing in absolute terms.     

APC/C-Cdc20-mediated degradation of cyclin B participates in CSF arrest in unfertilized Xenopus eggs

In vertebrates, unfertilized eggs are arrested at meiotic metaphase II (meta-II) by cytostatic factor (CSF), with Cdc2 activity maintained at a constant, high level. CSF is thought to suppress cyclin B degradation through the inhibition of the anaphase-promoting complex/cyclosome (APC/C)-Cdc20 while cyclin B synthesis continues in unfertilized eggs. Thus, it is a mystery how Cdc2 activity is kept constant during CSF arrest. Here, we show that the APC/C-Cdc20 can mediate cyclin B degradation in CSF-arrested Xenopus eggs and extracts, in such a way that when Cdc2 activity is elevated beyond a critical level, APC/C-Cdc20-dependent cyclin B degradation is activated and Cdc2 activity consequently declines to the critical level. This feedback control of Cdc2 activity is shown to be required for keeping Cdc2 activity constant during meta-II arrest. We have also shown that Mos/MAPK pathway is essential for preventing the cyclin B degradation from inactivating Cdc2 below the critical level required to sustain meta-II arrest. Our results indicate that under CSF arrest, Mos/MAPK activity suppresses cyclin B degradation, preventing Cdc2 activity from falling below normal meta-II levels, whereas activation of APC/C-Cdc20-mediated cyclin B degradation at elevated levels of Cdc2 activity prevents Cdc2 activity from reaching excessively high levels.     

Theoretical calculations of the helix–coil transition of DNA in the presence of large,cooperatively binding ligands

Theoretical calculations are conducted on the helix–coil transition of DNA, in the presence of large, cooperatively binding ligands modeled after the DNA-binding proteins of current biological interest. The ligands are allowed to bind both to helx and to coil, to cover up any number of bases or base pairs in the complex, and to interact cooperatively with their nearest neighbors. The DNA is treated in the infinite homogeneous Ising model approximation, and all calculations are done by Lifson's method of sequence-generating functions. DNA melting curves are calculated by computer in order to expolore the effects on the transition of ligand size, binding constant, free activity, and ligand–ligand cooperativity. The calculations indicate that (1) at the same intrinsic free energy change per base pair of the complexes, small ligands, for purely entropic reasons, are more effective than are large ligands in shifting the DNA melting temperature; (2) the response of the DNA melting temperature to increased ligand binding constant K and/or free ligand activity L is adequately represented at high values of KL (but not at low KL) by a simple independent site model; (3) if curves are calculated with the total amount of added ligand remaining constant and the free ligand activity allowed to vary throughout the transition, biphasic melting curves can be obtained in the complete absence of ligand–ligand cooperativity. In an Appendix, the denaturation of poly[d(A-T)] in the presence of the drug, netropsin, is used to verify some features of the theory and to illustrate how the theory can be used to obtain numerical estimates of the ligand binding parameters from the experimental melting curves.     

Acid Hydrolase Activity During Growth and Encystment in Acanthamoeba castellanii*

SYNOPSIS. The activity and sedimentation of acid phosphatase (APase), acid deoxyribonuclease (DNase), and acid ribonuclease (RNase) were investigated throughout growth and encystment in Acanthamoeba castellanii. The activities/mg protein of all 3 hydrolases are high in young cultures and decrease to constant levels in postlog cells. The RNase activity/ ameba decreases 50% during growth, whereas the activity/cell of both APase and DNase remains constant. The percent sedimentation at 20,000 g of all 3 enzymes gradually increases from about 40% in midlog to a plateau of 80% in postlog cells. During encystment, the sedimentation behavior of RNase differs from that of APase and DNase. Encystment is characterized by a differential decrease in the activity/cell of the 3 hydrolases, with RNase decreasing most rapidly and APase least rapidly. APase is unique in that a transient increase of its specific activity is noted during encystment, even though its activity/cell is decreasing.     

Variable threshold as a model for selective attention, (de)sensitization,and anesthesia in associative neural networks

We study the influence of a variable neuronal threshold on fixed points and convergence rates of an associative neural network in the presence of noise. We allow a random distribution in the activity levels of the patterns stored, and a modification to the standard Hebbian learning rule is proposed for this purpose. There is a threshold at which the retrieval ability, including the average final overlap and the convergence rate, is optimized for patterns with a particular activity level at a given noise level. This type of selective attention to one class of patterns with a certain activity level may be obtained at the cost of reducing the retrieval ability of the network for patterns with different activity levels. The effects of a constant threshold independent of noise, time, and pattern are discussed. For high-(low-) activity patterns, the average final overlap is shown to be increased at high noise levels and decreased at low noise levels by a negative (positive) constant threshold, whereas a positive (negative) threshold always reduces the final average overlap. When the magnitude of the constant threshold exceeds a critical value, there is no retrieval. Rates of convergence towards the stored pattern with negative (positive) thresholds are greater than those with positive (negative) thresholds. These results are related to (de)sensitization and anesthesia. For certain threshold values and patterns with certain activity levels, hysteresis appears in the plot of the average final overlap versus the noise level, even for first order interactions. We make the analogy between the pattern-dependent neuronal threshold proposed in the present paper and the task-related modulation in neuronal excitability determined by cognitive factors, such as the attentional state of a higher animal. A constant threshold is associated with overall changes in neuronal excitability caused, e.g., by various drugs and physical injuries. Neurophysiological evidence of a dynamically variable neuronal threshold, such as accommodation and potentiation, is presented.