Light-dependent regulation of cyanobacterial phytochrome expression.
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A histidine kinase protein (Cph1) with sequence homology and spectral characteristics very similar to those of the plant phytochrome has been recently identified in the cyanobacterium Synechocystis sp. strain PCC 6803. Cph1 together with Rcp1 (a protein homologue to the response regulator CheY) forms a light-regulated two-component system whose function is presently unknown. Levels of cph1 rcp1 mRNA increase in the dark and decrease upon reillumination. A dark-mediated increase in cph1 rcp1 mRNA levels was inhibited by the presence of glucose, but not by inhibition of the photosynthetic electron flow. The half-life of cph1 rcp1 transcript in the light was about fourfold shorter than in the dark, indicating that control of cph1 rcp1 transcript stability is one of the mechanisms by which light regulates expression of the cyanobacterial phytochrome. After 15 min of darkness, 3-min pulses of red, blue, green, and far-red light were equally efficient in decreasing the cph1 rcp1 mRNA levels. Red light downregulation was not reversed by far-red light, suggesting that cph1 rcp1 mRNA levels are not controlled by a phytochrome-like photoreceptor. Furthermore, a Synechocystis strain containing an H538R Cph1 point mutation, unable to phosphorylate Rcp1, shows normal light-dark regulation of the cph1 rcp1 transcript levels. Our data suggest a role of cyanobacterial phytochrome in the control of processes required for adaptation in light-dark and dark-light transitions. (+info)
A circadian clock regulates the pH of the fish retina.
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Although it is generally accepted that the acid/base ratio of tissue, as represented by the pH, is strictly regulated to maintain normal function, recent studies in the nervous system have shown that neuronal activity can result in significant shifts in pH. In the vertebrate retina, many cellular phenomena, including neuronal activity, are regulated by a circadian clock. We thus investigated whether a circadian clock regulates the pH of the retina. pH-sensitive microelectrodes were used to measure the extracellular pH of the in vitro goldfish retina superfused with a bicarbonate-based Ringer solution in the subjective day and night; that is, under conditions of constant darkness. These measurements demonstrated that a circadian clock regulates the pH of the vertebrate retina so that the pH is lower at night compared to the day. This day-night difference in retinal pH was observed at two different values of Ringer solution pH, indicating that the circadian phenomenon is independent of the superfusion conditions. The circadian-induced shift in pH was several times greater than light-induced pH changes and large enough to influence synaptic transmission between retinal neurons. These findings indicate that a circadian clock regulates the pH of the vertebrate retina. Thus, an intrinsic oscillator in neural tissue may modulate metabolic activity and pH as part of normal daily function. (+info)
A kinetic assessment of the sequence of electron transfer from F(X) to F(A) and further to F(B) in photosystem I: the value of the equilibrium constant between F(X) and F(A).
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The x-ray structure analysis of photosystem I (PS I) crystals at 4-A resolution (Schubert et al., 1997, J. Mol. Biol. 272:741-769) has revealed the distances between the three iron-sulfur clusters, labeled F(X), F(1), and F(2), which function on the acceptor side of PS I. There is a general consensus concerning the assignment of the F(X) cluster, which is bound to the PsaA and PsaB polypeptides that constitute the PS I core heterodimer. However, the correspondence between the acceptors labeled F(1) and F(2) on the electron density map and the F(A) and F(B) clusters defined by electron paramagnetic resonance (EPR) spectroscopy remains controversial. Two recent studies (Diaz-Quintana et al., 1998, Biochemistry. 37:3429-3439;, Vassiliev et al., 1998, Biophys. J. 74:2029-2035) provided evidence that F(A) is the cluster proximal to F(X), and F(B) is the cluster that donates electrons to ferredoxin. In this work, we provide a kinetic argument to support this assignment by estimating the rates of electron transfer between the iron-sulfur clusters F(X), F(A), and F(B). The experimentally determined kinetics of P700(+) dark relaxation in PS I complexes (both F(A) and F(B) are present), HgCl(2)-treated PS I complexes (devoid of F(B)), and P700-F(X) cores (devoid of both F(A) and F(B)) from Synechococcus sp. PCC 6301 are compared with the expected dependencies on the rate of electron transfer, based on the x-ray distances between the cofactors. The analysis, which takes into consideration the asymmetrical position of iron-sulfur clusters F(1) and F(2) relative to F(X), supports the F(X) --> F(A) --> F(B) --> Fd sequence of electron transfer on the acceptor side of PS I. Based on this sequence of electron transfer and on the observed kinetics of P700(+) reduction and F(X)(-) oxidation, we estimate the equilibrium constant of electron transfer between F(X) and F(A) at room temperature to be approximately 47. The value of this equilibrium constant is discussed in the context of the midpoint potentials of F(X) and F(A), as determined by low-temperature EPR spectroscopy. (+info)
The transcription factor DBP affects circadian sleep consolidation and rhythmic EEG activity.
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Albumin D-binding protein (DBP) is a PAR leucine zipper transcription factor that is expressed according to a robust circadian rhythm in the suprachiasmatic nuclei, harboring the circadian master clock, and in most peripheral tissues. Mice lacking DBP display a shorter circadian period in locomotor activity and are less active. Thus, although DBP is not essential for circadian rhythm generation, it does modulate important clock outputs. We studied the role of DBP in the circadian and homeostatic aspects of sleep regulation by comparing DBP deficient mice (dbp-/-) with their isogenic controls (dbp+/+) under light-dark (LD) and constant-dark (DD) baseline conditions, as well as after sleep loss. Whereas total sleep duration was similar in both genotypes, the amplitude of the circadian modulation of sleep time, as well as the consolidation of sleep episodes, was reduced in dbp-/- under both LD and DD conditions. Quantitative EEG analysis demonstrated a marked reduction in the amplitude of the sleep-wake-dependent changes in slow-wave sleep delta power and an increase in hippocampal theta peak frequency in dbp-/- mice. The sleep deprivation-induced compensatory rebound of EEG delta power was similar in both genotypes. In contrast, the rebound in paradoxical sleep was significant in dbp+/+ mice only. It is concluded that the transcriptional regulatory protein DBP modulates circadian and homeostatic aspects of sleep regulation. (+info)
GABA(C) receptors control adaptive changes in a glycinergic inhibitory pathway in salamander retina.
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We studied the role of GABA in adaptive changes in a lateral inhibitory system in the tiger salamander retina. In dark-adapted retinal slice preparations picrotoxin caused a slow enhancement of glycine-mediated IPSCs in ganglion cells. The enhancement of glycinergic IPSCs developed slowly over the course of 5-20 min, even though picrotoxin blocked both GABA(A) and GABA(C) receptors within a few seconds. The slow enhancement of glycinergic IPSCs by picrotoxin was much weaker in light-adapted preparations. The slow enhancement of glycinergic inhibitory inputs was not produced by bicuculline, indicating that it involved GABA(C) receptors. The responses of ganglion cells to direct application of glycine were not enhanced by picrotoxin, indicating that the enhancement was not caused by an action on glycine receptors. In dark-adapted eyecup preparations picrotoxin caused a slow enhancement of glycinergic IPSPs and transient lateral inhibition produced by a rotating windmill pattern, similar to the effect of light adaptation. The results suggest that the glycinergic inhibitory inputs are modulated by an unknown substance whose synthesis and/or release is inhibited in dark-adapted retinas by GABA acting at GABA(C) receptors. (+info)
ABI3 affects plastid differentiation in dark-grown Arabidopsis seedlings.
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The Arabidopsis ABSCISIC ACID-INSENSITIVE3 (ABI3) protein has been identified previously as a crucial regulator of late seed development. Here, we show that dark-grown abi3 plants, or abi3 plants returned to the dark after germination in the light, developed and maintained an etioplast with a prominent prolamellar body at developmental stages in which the wild type did not. Overexpression of ABI3 led to the preservation of the plastid ultrastructure that was present at the onset of darkness. These observations suggest that ABI3 plays a role in plastid differentiation pathways in vegetative tissues. Furthermore, the analysis of deetiolated (det1) abi3 double mutants revealed that DET1 and ABI3 impinge on a multitude of common processes. During seed maturation, ABI3 required DET1 to achieve its full expression. Mature det1 abi3 seeds were found to be in a highly germinative state, indicating that germination is controlled by both DET1 and ABI3. During plastid differentiation in leaves of dark-grown plants, DET1 is required for the action of ABI3 as it is during seed development. Together, the results suggest that ABI3 is at least partly regulated by light. (+info)
Mutagenesis and functional characterization of the glnB, glnA, and nifA genes from the photosynthetic bacterium Rhodospirillum rubrum.
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Nitrogen fixation is tightly regulated in Rhodospirillum rubrum at two different levels: transcriptional regulation of nif expression and posttranslational regulation of dinitrogenase reductase by reversible ADP-ribosylation catalyzed by the DRAT-DRAG (dinitrogenase reductase ADP-ribosyltransferase-dinitrogenase reductase-activating glycohydrolase) system. We report here the characterization of glnB, glnA, and nifA mutants and studies of their relationship to the regulation of nitrogen fixation. Two mutants which affect glnB (structural gene for P(II)) were constructed. While P(II)-Y51F showed a lower nitrogenase activity than that of wild type, a P(II) deletion mutant showed very little nif expression. This effect of P(II) on nif expression is apparently the result of a requirement of P(II) for NifA activation, whose activity is regulated by NH(4)(+) in R. rubrum. The modification of glutamine synthetase (GS) in these glnB mutants appears to be similar to that seen in wild type, suggesting that a paralog of P(II) might exist in R. rubrum and regulate the modification of GS. P(II) also appears to be involved in the regulation of DRAT activity, since an altered response to NH(4)(+) was found in a mutant expressing P(II)-Y51F. The adenylylation of GS plays no significant role in nif expression or the ADP-ribosylation of dinitrogenase reductase, since a mutant expressing GS-Y398F showed normal nitrogenase activity and normal modification of dinitrogenase reductase in response to NH(4)(+) and darkness treatments. (+info)
CSF 5-HIAA and nighttime activity in free-ranging primates.
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Men with low CNS serotonin turnover, as measured by cerebrospinal fluid 5-hydroxyindoleacetic acid (CSF 5-HIAA) concentrations, exhibit aberrant circadian activity patterns characterized by disrupted sleep rhythms and daytime hyperactivity. To assess whether similar patterns are found in nonhuman primates we examined the relationships between CSF 5-HIAA and nighttime activity in free-ranging monkeys. CSF samples were obtained from 16 adult male rhesus macaques living on a 475 acre, heavily forested sea island. Each subject was captured, fitted with a radio-telemetry motion-detector collar, and then released back into its group. A receiver placed near the sleeping trees of the study subjects recorded activity between 2100 hrs and 0600 hrs. Trained observers recorded behavioral data during the day. The animals followed a typical diurnal activity pattern, as they were active 74% of the sampled time during the day and 37% of the sampled time during the night. CSF 5-HIAA concentrations were inversely correlated with total duration of nighttime activity as well as mean duration of all active events. Nighttime activity was inversely correlated with daytime activity. CSF 3-methoxy-hydroxyphenylglycol (MHPG) concentrations were positively correlated with total nighttime activity, and inversely correlated with daytime sleep frequency. We conclude that male rhesus with low CSF 5-HIAA concentrations have higher total nighttime activity, longer mean periods of nighttime activity, and sleep more during the day than do males with high CSF 5-HIAA concentrations. This suggests that low serotonergic neurotransmission is associated with aberrant diurnal activity, as evidenced by a disruption of nighttime sleep patterns and a compensatory higher rate of inactivity during the day. (+info)