Oxidation-reduction potential dependence of low-temperature photoreactions of chloroplast Photosystem II

The light-induced free-radical signal of Photosystem II (observed after illumination at 77 °K) has been studied in chloroplasts as a function of the oxidation-reduction potential established prior to freezing. The intensity of the light-induced signal is unchanged in the potential region of +590 mV to +760 mV. At higher potential (+850 mV), there is a 30% decrease in signal intensity. The light-induced signal decreases to zero in the low-potential region, with a midpoint potential of +475 mV. These results are considered in terms of a Photosystem II reaction-center complex in which the light-induced free-radical signal arises from the oxidized form of the reaction-center chlorophyll, and this chlorophyll molecule is capable of being reduced at liquid-nitrogen temperature by a secondary electron donor which has a midpoint oxidation-reduction potential of +475 mV.     

Purification and properties of a germination-specific cortex-lytic enzyme from spores of Bacillus megaterium KM.

Two peptidoglycan-lytic enzyme activities were isolated from spores of Bacillus megaterium KM. Surface-bound lytic enzyme was extracted from dormant spores and hydrolysed a variety of peptidoglycan substrates including isolated spore cortex, but did not cause refractility changes in permeabilized spores. Germination-specific lytic enzyme activity appeared early in germination and had minimal activity on isolated peptidoglycan substrates, but caused refractility changes in permeabilized spores of several Bacillus isolated peptidoglycan substrates, but caused refractility changes in permeabilized spores of several Bacillus species. The germination-specific lytic enzyme was shown to be a heat-sensitive 29 kDa protein with maximal activity at pH 6.5. It catalysed post-commitment muramic acid delta-lactam synthesis and displayed an inhibitor profile similar to that for post-commitment A600 loss. The relationship of the germination-specific enzyme to a recently proposed model of spore germination is discussed.     

Differential control of proto-oncogene c-myc and c-fos expression in lymphocytes and fibroblasts.

In lymphocytes stimulated with the mitogen phytohaemagglutinin, an inhibitor of the enzyme ADP-ribosyltransferase (ADPRT) completely blocks the proliferative response and the increase in expression of the proto-oncogene c-myc without affecting c-fos significantly. Conversely, in fibroblasts the serum-induced growth is not affected by the ADPRT inhibitor, and both oncogenes are dramatically super-induced. Hence there are differences between lymphocyte and fibroblast early responses to mitogenic stimulation and also between regulation of c-fos and c-myc gene expression.     

The influence of ration size and feeding frequency on ammonia excretion by juvenile Atlantic cod, Gadus morhua L.

Small groups of juvenile Atlantic cod, Gadus morhua L., were kept at 14°C in through-flow tanks and were fed known quantities of a compounded diet of natural food. The cod were fed single and multiple meals with ration size in the range 0.5 to 4.1% of total wet fish body weight. Ammonia production in each feeding experiment was monitored continuously.
For single-meal experiments, significant relationships were derived between ration size and (a) total ammonia excreted, (b) total exogenous ammonia excreted above endogenous excretion levels, (c) duration of the elevated phase of ammonia excretion, (d) maximum rate of ammonia excretion, and (e) time delay after feeding to reach maximum rate of ammonia excretion. Relationships between nitrogen loss as ammonia and nitrogen intake were examined and estimates of endogenous excretion rate and maintenance ration made.
Repetitive feeding resulted in cyclical variation in ammonia excretion. At the lowest ration size, ammonia excretion rates had nearly returned to the pre-feeding level within 24 h. At higher feeding levels, the effect of each successive meal tended to be cumulative, resulting in increasingly higher ammonia excretion rates which only stabilized towards the end of the experiments.     

Nucleic acid-like structures III. Oligomerization of 3′-deoxyadenosine 2′,5′-diphosphoimidazolide

Summary The activated dimonophosphate of 3-deoxyadenosine (cordycepin) undergoes oligomerization to produce a new family of pyrophosphate-linked oligomers in which the average repeating unit involves a nine-atom structural group. The presence of a poly(U) template increase the relative yields of higher oligomers, although the template-free reaction is itself extremely efficient.For the previous paper in this series see Schwartz et al. (1987)     

Effects of α2-Adrenergic Agonists and Antagonists on Photoreceptor Membrane Currents

Type B photoreceptors of the nudibranch mollusc Hermissenda crassicornis receive excitatory synaptic potentials (EPSPs) whose frequency is controlled by potential changes of a neighboring cell known as the S optic ganglion cell which is thought to be electrically coupled to the presynaptic source of these EPSPs, the E optic ganglion cell. The frequency of the EPSPs increases when a conditioned stimulus (light) is paired with an unconditioned stimulus (rotation) during acquisition of a Pavlovian conditioned response. The results of the present study are consistent with an adrenergic origin for these EPSPs. Noradrenergic agonists (greater than 100 microM), norepinephrine and clonidine, only slightly depolarize the type B cell but clearly prolong its depolarizing response to light. Serotonin, by contrast, causes hyperpolarization of the type B cell's resting potential as well as after a light step. Clonidine reduces voltage-dependent outward K+ currents (IA, an early current, ICa2+-K+, a late Ca2+-dependent current) that control the type B cell's excitability (and thus its light response and membrane potential). These effects of clonidine are reduced or blocked by the alpha 2-receptor antagonist, yohimbine (0.5 microM), but not the alpha 1-blocker, prazosin. The same yohimbine concentration also blocked depolarizing synaptic excitation of the type B cell in response to depolarization of a simultaneously impaled S optic ganglion cell. Histochemical techniques (both the glyoxylic acid method of de la Torre and Surgeon and the formaldehyde-induced fluorescence or Falck-Hillarp method) demonstrated the presence of a biogenic amine(s) within a single neuron in each optic ganglion as well as three or four cells within the vicinity of previously identified visual interneurons. No serotonergic neurons were found within the optic ganglion or in proximity to visual interneurons. A clonidine-like synaptic effect on type B cells, therefore, could amplify conditioning-specific changes of membrane currents by increasing type B depolarization and possibly, as well, by elevating intracellular second messengers.     

Effect of differential gastric evacuation and multispecies prey items on estimates of daily energy intake in juvenile chinook salmon

Synopsis The caloric density of stomach contents in juvenile chinook salmon,Oncorhynchus tshawytscha, was not affected by gastric evacuation, suggesting a constant caloric density of stomach contents during evacuation. Differences in the caloric density of prey consumed did affect caloric density of stomach contents over a 24-h period. Consumption of the amphipodCorophium sp. was associated with reduced caloric densities of stomach contents. During periods whenCorophium contributed more than 4% of the stomach contents, average caloric density declined from 5.56 to 5.33 kcal g^–1. Despite this difference, estimates of daily energy intake of juvenile chinook salmon were only 3%, greater when developed from the mean caloric density of stomach contents excludingCorophium.