SEARCH · PubMed Health

Results for “segmentation clock”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 307 records · Page 17Linked to original sources

PubMed2006

Mitral valve periprosthetic leakage: Anatomical observations in 135 patients from a multicentre study.

OBJECTIVE: Prosthetic valve dysfunction after mitral valve replacement (MVR) may be caused by several factors, which often lead to repeated surgery. One of the most frequent determinants of reoperation is periprosthetic leakage (PPL). A few published reports have analysed PPL incidence and postoperative results after MVR, but no specific attention has been paid towards the potential relation between anatomical factors and PPL occurrence, particularly not bacterial-related. The aim of this study was to evaluate the location of PPL after MVR through a multicentre retrospective study. METHODS: Between January 1985 and November 2005, 135 patients underwent reoperation at four institutions because of PPL after MVR and met the study inclusion criteria. The mitral valve annulus (MVA) was analysed in a clockwise format, indicating 12 o'clock as the mid-point of anterior annulus as viewed from the atrium. RESULTS: Overall hospital mortality was 3.7% (five patients). Repair of PPL was carried out in 83 cases whereas prosthetic valve replacement was necessary in 52 cases. The total number of sectors involved in PPL was 244. PPL occurred more frequently between hour 5 and hour 6, and hour 10 and hour 11, with the risk of leakage being, 2.8 and 2.0 times higher, respectively, than in any other portion of the MVA. CONCLUSIONS: Our study suggests that PPL occurs more frequently at antero-lateral and postero-medial segments of MVA. This finding might be linked to unusual anatomical and functional factors of the MVA and may call for adjunctive care to these sectors of MVA when performing suture placement during MVR.

Aged↗

PubMed2005

Neuronal control of leech behavior.

The medicinal leech has served as an important experimental preparation for neuroscience research since the late 19th century. Initial anatomical and developmental studies dating back more than 100 years ago were followed by behavioral and electrophysiological investigations in the first half of the 20th century. More recently, intense studies of the neuronal mechanisms underlying leech movements have resulted in detailed descriptions of six behaviors described in this review; namely, heartbeat, local bending, shortening, swimming, crawling, and feeding. Neuroethological studies in leeches are particularly tractable because the CNS is distributed and metameric, with only 400 identifiable, mostly paired neurons in segmental ganglia. An interesting, yet limited, set of discrete movements allows students of leech behavior not only to describe the underlying neuronal circuits, but also interactions among circuits and behaviors. This review provides descriptions of six behaviors including their origins within neuronal circuits, their modification by feedback loops and neuromodulators, and interactions between circuits underlying with these behaviors.

Animals↗

PubMed2001

Synchrony of rest tremor in multiple limbs in parkinson's disease: evidence for multiple oscillators.

Recent evidence points to involvement of central nervous system oscillators in Parkinson's disease (PD) rest tremor. It remains unknown whether one or multiple oscillators cause tremor in multiple limbs. Based on the prediction that multiple oscillators would cause low coherence even with similar average frequency, we studied 22 PD patients using accelerometers on multiple limbs. Records were digitized and spectral analysis was performed. Peak frequencies in the arms, legs, and chin were similar, indicating that biomechanical factors did not determine the frequency. Coherence between different axes of individual accelerometers and between different segments of the same limb was high. However, coherence between tremor in different limbs was low. There was no consistent pattern across patients of ipsi- vs. contralateral predominance of coherence. These data suggest that tremor in PD is generated by multiple oscillatory circuits, which operate on similar frequencies.

Aged↗

PubMed2003

Binding and segmentation of multiple objects through neural oscillators inhibited by contour information.

Temporal correlation of neuronal activity has been suggested as a criterion for multiple object recognition. In this work, a two-dimensional network of simplified Wilson-Cowan oscillators is used to manage the binding and segmentation problem of a visual scene according to the connectedness Gestalt criterion. Binding is achieved via original coupling terms that link excitatory units to both excitatory and inhibitory units of adjacent neurons. These local coupling terms are time independent, i.e., they do not require Hebbian learning during the simulations. Segmentation is realized by a two-layer processing of the visual image. The first layer extracts all object contours from the image by means of "retinal cells" with an "on-center" receptive field. Information on contour is used to selectively inhibit Wilson-Cowan oscillators in the second layer, thus realizing a strong separation among neurons in different objects. Accidental synchronism between oscillations in different objects is prevented with the use of a global inhibitor, i.e., a global neuron that computes the overall activity in the Wilson-Cowan network and sends back an inhibitory signal. Simulations performed in a 50 x 50 neural grid with 21 different visual scenes (containing up to eight objects + background) with random initial conditions demonstrate that the network can correctly segment objects in almost 100% of cases using a single set of parameters, i.e., without the need to adjust parameters from one visual scene to the next. The network is robust with reference to dynamical noise superimposed on oscillatory neurons. Moreover, the network can segment both black objects on white background and vice versa and is able to deal with the problem of "fragmentation."The main limitation of the network is its sensitivity to static noise superimposed on the objects. Overcoming this problem requires implementation of more robust mechanisms for contour enhancement in the first layer in agreement with mechanisms actually realized in the visual cortex.

Artifacts↗

PubMed1996

Retinopathy of prematurity: guidelines for screening and treatment. The report of a Joint Working Party of The Royal College of Ophthalmologists and the British Association of Perinatal Medicine.

Following the report that severe retinopathy of prematurity (ROP) can be treated effectively, a working party of the Royal College of Ophthalmologists and the British Association of Perinatal Medicine was convened in 1990 to draw up guidelines for screening [1]. These have been revised by a reconvened working party (Appendix D) and the document expanded to include other relevant issues such as the practicalities of treatment, counselling and end-stage retinopathy. Screening is recommended for all babies at risk of severe ROP, i.e. those of birth weight < or = 1500 g and or < or = 31 weeks gestational age. The aim of screening is to identify severe ROP (stage 3) which may require treatment, or, in a baby due to be discharged to home or to another hospital, any ROP which has the potential to become severe. The first examination should be between 6 and 7 weeks postnatal age, and subsequent examinations continued until vascularisation has progressed into zone 3 when the risk of stage 3 ROP has passed. As the window of time available for treatment is very short, examinations should be undertaken every 2 weeks. Treatment is currently undertaken when threshold stage has been reached. Threshold ROP is defined as: Stage 3 ROP: involving five or more contiguous or eight or more cumulative clock hours; in the presence of congestion of the posterior pole vessels--'plus' disease. Treatment can be by cryotherapy or laser in the neonatal unit. Parents have a right to know what may befall their baby, and it is important to provide balanced information, and this should be given to parents of babies: at risk of developing any ROP; who are close to or have severe disease or requiring treatment; blinded by ROP. Despite meticulous clinical care, babies are occasionally blinded by ROP. That is not the end of the road and there is still work for the clinician to do: counsel on the role of vitrectomy; manage the disorganised anterior segment; initiate and actively participate in the care of the visually impaired child. The screening and management should be undertaken or supervised by senior ophthalmologists with a specific interest in this condition. It is recommended that one or two consultants in each area should gain expertise and supervise training. Referencing here is not exhaustive with preference being given to recent reviews. With certain exceptions citations are at the commencement of each section rather than at specific issues.

Humans↗

PubMed1997

[Block excision with tectonic corneoscleroplasty for cystic and/or diffuse epithelial invasion of the anterior eye segment. Report of 51 consecutive patients (1980-1996)].

PURPOSE: Many surgical and nonsurgical techniques for the treatment of cystic and/or diffuse sheet-like epithelial ingrowth of the anterior chamber led to recurrences or even enucleation of the eye. PATIENTS AND METHODS: There were 32 (62.7%) men and 19 (37.3%) women ranging in age from 1 month to 80 years (median, 55.5 years). Cystic epithelial ingrowth occurred in 45 patients, and diffuse sheet-like epithelial downgrowth in 6 patients. Block excision consisted of simultaneous en bloc removal of epithelial ingrowth together with adjacent iris, pars plicata of the ciliary body, and cornea and sclera in full-thickness. The resulting defect was covered with a tectonic corneoscleral graft. The median follow-up was 7.9 years. RESULTS: The main causes for epithelial ingrowth were trauma (41.2%) and complicated cataract extractions (27.5%). Eleven patients had undergone surgery of epithelial ingrowth before block excision, elsewhere. Epithelial ingrowth involved up to 5 clock hours of the circumference of the chamber angle (median, 3 clock hours). The median preoperative visual acuity was 0.3 (range, hand motions -1.0). The median diameter of the block excision was 8.0 mm (range, 5.5-12.0 mm). The main postoperative complications were vitreous hemorrhage (27.5%) and corneal endothelial decompensation (21.6%). The median postoperative visual acuity was 0.2. Visual acuity was > or = 0.3 in 43.1% of patients and < 0.1 in 35.3% of patients. Visual results were significantly better after simultaneous cataract extraction with intraocular lens implantation (n = 5). Histopathologically, the invading epithelium mainly consisted of nonkeratinizing squamous epithelium with goblet cells (64.7%). There was a secondary proliferation of corneal endothelium on the cyst's surface in 82.4% of patients. There was no clinical evidence of recurrence of epithelial ingrowth, and no enucleation had to be performed during follow-up. CONCLUSION: Block excision with tectonic corneoscleral grafting is the treatment of choice for cystic and/or diffuse sheet-like epithelial ingrowth of the anterior chamber or anterior uvea, retrospectively.

Adolescent↗

PubMedDate not supplied

Influence of light-dark cycle alteration on free radical level in rat cns.

The experiment analysed the evolution of several redox parameters caused by light-dark cycle alteration in different rat brain segments: cortex, brain stem, diencephalon. Continuous light (L:L) enhanced SOD, CAT, GPX and GSH levels to different extents in the various central nervous system regions investigated, depending on the entrainment period: 24 h, 48 h, 72 h, 7 days. Continuous darkness (D:D -24 h, 48 h, 72 h, 7 days) generally diminished SOD, CAT and GSH values, but increased GPX at 48 h and CAT at 72 h. MDA levels were higher in D:D, too. After 7 days L:L or D:D redox indices tended to reach the normal range again. Redox system spatio-temporal versatility and complementarity suggested the existence of a possible biochemical radical cell clock gear.

Animals↗

PubMed1999

Imaging of multicellular large-scale rhythmic calcium waves during zebrafish gastrulation.

Oscillations of cytosolic free calcium levels have been shown to influence gene regulation and cell differentiation in a variety of model systems. Intercellular calcium waves thus present a plausible mechanism for coordinating cellular processes during embryogenesis. Herein we report use of aequorin and a photon imaging microscope to directly observe a rhythmic series of intercellular calcium waves that circumnavigate zebrafish embryos over a 10-h period during gastrulation and axial segmentation. These waves first appeared at about 65% epiboly and continued to arise every 5-10 min up to at least the 16-somite stage. The waves originated from loci of high calcium activity bordering the blastoderm margin. Several initiating loci were active early in the wave series, whereas later a dorsal marginal midline locus predominated. On completion of epiboly, the dorsal locus was incorporated into the developing tail bud and continued to generate calcium waves. The locations and timing at which calcium dynamics are most active appear to correspond closely to embryonic cellular and syncytial sites of known morphogenetic importance. The observations suggest that a panembryonic calcium signaling system operating in a clock-like fashion might play a role during vertebrate axial patterning.

Aequorin↗

PubMed2006

Non-linear dynamic analysis of the cardiac rhythm during transient myocardial ischemia.

Coronary artery occlusions related to myocardial ischemia drive cardiac control system reactions that may lead to heart failure. The purpose of this study was to assess the autonomic nervous system (ANS) response during prolonged percutaneous transluminal coronary angioplasty (PTCA). Continuous ECG data were acquired from 50 patients before and during PTCA, with occlusions in the left anterior descending, left circumflex or right coronary artery. Heart rate variability (HRV) was analyzed for 3-min segments of the R-R interval signal obtained from ECG data. The ANS behavior was evaluated by HRV analysis using fractal-like indices. The fractal scalar exponent alpha(1) and power-law slope beta decreased considerably during PTCA. This indicates that significant reactions of autonomic control of the heart rate occurred during coronary artery occlusions, with a reduction in complexity of the ANS.

Adult↗

PubMed2005

Utility of nuclear DNA intron markers at lower taxonomic levels: phylogenetic resolution among nine Tragelaphus spp.

Phylogenetic relationships among the nine spiral-horn antelope species of the African bovid tribe Tragelaphini are controversial. In particular, mitochondrial DNA sequencing studies are not congruent with previous morphological investigations. To test the utility of nuclear DNA intron markers at lower taxonomic levels and to provide additional data pertinent to tragelaphid evolution, we sequenced four nuclear DNA segments (MGF, PRKCI, SPTBN, and THY) and combined these data with mitochondrial DNA sequences from three genes (cytochrome b, 12S rRNA, and 16S rRNA). Our molecular supermatrix comprised 4682 characters which were analyzed independently and in combination. Parsimony and model based phylogenetic analyses of the combined nuclear DNA data are congruent with those derived from the analysis of mitochondrial gene sequences. The corroboration between nuclear and mtDNA gene trees reject the possibility that genetic processes such as lineage sorting, gene duplication/deletion and hybrid speciation account for the conflict evident in the previously published phylogenies. It suggests rather that the morphological characters used to delimit the Tragelaphid species are subject to convergent evolution. Divergence times among species, calculated using a relaxed Bayesian molecular clock, are consistent with hypotheses proposing that climatic oscillations and their impact on habitats were the major forces driving speciation in the tribe Tragelaphini.

Animals↗

PubMed2006

Neocortical network activity in vivo is generated through a dynamic balance of excitation and inhibition.

The recurrent excitatory and inhibitory connections between and within layers of the cerebral cortex are fundamental to the operation of local cortical circuits. Models of cortical function often assume that recurrent excitation and inhibition are balanced, and we recently demonstrated that spontaneous network activity in vitro contains a precise balance of excitation and inhibition; however, the existence of a balance between excitation and inhibition in the intact and spontaneously active cerebral cortex has not been directly tested. We examined this hypothesis in the prefrontal cortex in vivo, during the slow (<1 Hz) oscillation in ketamine-xylazine-anesthetized ferrets. We measured persistent network activity (Up states) with extracellular multiple unit and local field potential recording, while simultaneously recording synaptic currents in nearby cells. We determined the reversal potential and conductance change over time during Up states and found that the body of Up state activity exhibited a steady reversal potential (-37 mV on average) for hundreds of milliseconds, even during substantial (21 nS on average) changes in membrane conductance. Furthermore, we found that both the initial and final segments of the Up state were characterized by significantly more depolarized reversal potentials and concomitant increases in excitatory conductance, compared with the stable middle portions of Up states. This ongoing temporal evolution between excitation and inhibition, which exhibits remarkable proportionality within and across neurons in active local networks, may allow for rapid transitions between relatively stable network states, permitting the modulation of neuronal responsiveness in a behaviorally relevant manner.

Action Potentials↗

PubMed2006

Modelling long-term heart rate variability: an ARFIMA approach.

Long-term heart rate variability (HRV) series can be described by time-variant autoregressive modelling. HRV recordings show dependence between distant observations that is not negligible, suggesting the existence of long-range correlations. In this work, selective adaptive segmentation combined with fractionally integrated autoregressive moving-average models is used to capture long memory in HRV recordings. This approach leads to an improved description of the low- and high-frequency components in HRV spectral analysis. Moreover, it is found that in the 24-h recording of a case report, the long-memory parameter presents a circadian variation, with different regimes for day and night periods.

Adaptation, Physiological↗

PubMed2005

Head motion in humans alternating between straight and curved walking path: combination of stabilizing and anticipatory orienting mechanisms.

Anticipatory head orientation relative to walking direction was investigated in humans. Subjects were asked to walk along a 20 m perimeter, figure of eight. The geometry of this path required subjects to steer their body according to both curvature variations (alternate straight with curved walking) and walking direction (clock wise and counter clock wise). In agreement with previous results obtained during different locomotor tasks [R. Grasso, S. Glasauer, Y. Takei, A. Berthoz, The predictive brain: anticipatory control of head direction for the steering of locomotion, NeuroReport 7 (1996) 1170-1174; R. Grasso, P. Prevost, Y.P. Ivanenko, A. Berthoz, Eye-head coordination for the steering of locomotion in humans: an anticipatory synergy, Neurosci. Lett. 253 (2) (1998) 115-118; T. Imai, S.T. Moore, T. Raphan, B. Cohen, Interaction of body, head, and eyes during walking and turning, Exp. Brain Res. 136 (2001) 1-18; P. Prevost, Y. Ivanenko, R. Grasso, A. Berthoz, Spatial invariance in anticipatory orienting behaviour during human navigation, Neurosci. Lett. 339 (2002) 243-247; G. Courtine, M. Schieppati, Human walking along a curved path. I. Body trajectory, segment orientation and the effect of vision, Eur. J. Neurosci. 18 (2003) 177-190], the head turned toward the future walking direction. This anticipatory head behaviour was continuously modulated by the geometrical variations of the path. Two main components were observed in the anticipatory head behaviour. One was related to the geometrical form of the path, the other to the transfer of body mass from one foot to the other during stepping. A clear modulation of the head deviation pattern was observed between walking on curved versus straight parts of the path: head orientation was influenced to a lesser extent by step alternation for curved path where a transient head fixation was observed. We also observed good symmetry in the head deviation profile, i.e. the head tended to anticipate the future walking direction with the same amplitude when turning to the left (29.75 +/- 7.41 degrees of maximum head deviation) or to the right (30.86 +/- 9.92 degrees ). These findings suggest a combination of motor strategies underlying head stabilization in space and more global orienting mechanisms for steering the whole body in the desired direction.

Adult↗

PubMed1995

[A 75-year-old man with parkinsonism and sudden death].

We report a 75-year-old man with parkinsonism who died suddenly. The patient was well until 64 years of the age when he had an onset of tremor in his left hand. He was treated with a medicine in another hospital, and his tremor subsided. Five years after the onset, he started to note difficulty in fine finger movements and gait disturbance. He tended to lean backward with frequent falls. He was treated with bromocriptine, trihexyphenydil, and L-dops without apparent improvement. He visited our out patient clinic on November 11, 1993 when he was 75 years of the age. Neurologic examination at that time revealed an alert and well oriented man in no acute distress. Higher cerebral functions were intact. In the cranial nerves, he showed restriction in the upward as well as down ward gaze (40% of normal). He showed masking of the face and spoke in small voice. He walked in a stooped posture with small steps; retropulsion was present. Muscle rigidity was moderately positive in the neck, however, no rigidity was noted in the limbs. No abnormal involuntary movements were seen. He showed moderate bradykinesia and difficulty in finger tapping. Muscle stretch reflexes were normally elicited and the plantar response was flexor bilaterally. Sensation was intact. The autonomic nervous system appeared intact. He was treated with 300 mg/day of Sinemet with marginal improvement in his balance. In February 4, 1994, he had a common cold. On the next day, his parkinsonism worsened and he became unable to walk by himself. He was found unconscious in the bathroom on the same day. He was brought to our hospital by an ambulance. Upon arrival, he was unresponsive and was not breathing. Blood pressure could not be measured. Pupils were dilated without reaction to light. Cardiac resuscitation was attempted, however, ventricular fibrillation appeared on an EEG monitor, and he was pronounced dead at eleven o'clock in the morning. The patient was discussed in a neurological CPC, and the chief discussant arrived at the conclusion that the patient had progressive supranuclear palsy because of vertical gaze palsy, axial rigidity, and poor response to levodopa. Regarding the cause of his sudden death, the chief discussant thought that he developed pulmonary embolism. Postmortem examination revealed non-bacterial thrombotic endocarditis in the heart, but this did not appeared to be related to his sudden death. Multiple disseminated small emboli were found occluding small arteries of the left lung; this was consistent with acute pulmonary embolism, and this was thought to be the cause of his sudden death. In the central nervous system, marked atrophy of the globus pallidus was noted; both internal as well as external segments showed marked atrophy; no myelinated fibers were seen in the globus pallidus. Neuronal cell loss was marked in the globus pallidus, the subthalamic nucleus, and the substantia nigra. No Lewy bodies or tangles were seen. The histologic diagnosis was consistent with pallido-nigro-luysian atrophy. Brownish pigments such as seen in Hallervorden-Spatz disease were seen in the globus pallidus. In addition, formy spheroids were seen in the substantia nigra. However, iron deposits were not so strong as to suggest Hallervorden-Spatz disease. Pallido-nigro-luysian atrophy is a rare neurodegenerative disorder. It is interesting to note that this condition may mimic progressive supranuclear palsy or pure akinesia clinically.

Aged↗

PubMed2005

Evolutionary relationships of the limnochromini, a tribe of benthic deepwater cichlid fish endemic to Lake Tanganyika, East Africa.

Lake Tanganyika harbors an enormous diversity of cichlid fish that stem from eight distinct ancestral lineages, which colonized the lake after its formation 9 to 12 million years ago. Six of twelve currently described tribes are assigned to the "H-lineage," an assemblage of exclusively mouthbrood-ing cichlids, all of which evolved during a short period of time during the course of the primary radiation of lacustrine species. Our study focuses on the deepwater tribe Limnochromini, comprising bi-parental mouthbrooders, and is based on phylogenetic analysis of two mitochondrial gene segments. We confirm the polyphyletic origin of the Limnochromini as they are defined to date, in that Gnathochromis pfefferi is placed among the Tropheini, whereas the genus Benthochromis is presented as an independent lineage. The remaining nine species were unambiguously resolved as monophyletic and should be redefined as the tribe Limnochromini. Concerning generic assignments, the genus Greenwoodochromis appeared as monophyletic, Limnochromis as paraphyletic, and the genera Reganochromis and Baileychromis as monophyletic sister genera. The linearized tree analysis and the comparison of average sequence divergences to that of the remaining tribes of the H-lineage revealed a relatively recent but simultaneous proliferation of the Limnochromini, suggesting that the same environmental changes triggered the radiation of particular deepwater, benthic, pelagic, and littoral lineages. By using a preliminary calibration of a molecular clock based on gamma-corrected amino acid distances of the NADH2 gene, the diversification of the Limnochromini could tentatively be dated to 2.9-3.5 MYA, coinciding with a period of aridification in East Africa between 2.5 and 3 MYA. The lack of geographic color morphs and the structural uniformity and resource scarcity of deepwater habitats suggest that competition and resource partitioning leading to differential trophic specialization promoted speciation within the Limnochromini, rather than an allopatric model.

Animals↗

PubMed1996

Iris melanomas. Diagnostic problems.

PURPOSE: To illustrate some management problems with pigmented iris tumors. METHODS: This is a retrospective analysis of 234 iris tumors managed by one of the authors. Five patients were selected whose clinical course and pathologic data illustrate diagnostic problems that can occur with these tumors. RESULTS: In the patients described, diagnostic problems were noted. In two patients, glaucoma specialists were unable to correctly diagnose an iris melanoma before filtration. One patient had a 3-clock-hour iris biopsy that was not diagnostic for malignancy, yet the enucleated eye showed melanoma. One patient with a clinically stable iris tumor had a mixed-cell melanoma. CONCLUSIONS: Most iris-pigmented tumors have a benign disease course. In some case, despite little clinical evidence of disease activity or progression, a melanoma, often with epithelioid cells, may be present and neither a fine-needle nor an open biopsy is always diagnostic.

Adult↗

PubMed2005

Double average parallel steady-state free precession imaging: optimized eddy current and transient oscillation compensation.

Balanced steady-state free precession (SSFP) imaging is sensitive to off-resonance effects, which can lead to considerable artifacts during a transient phase following magnetization preparation or steady-state interruption. In addition, nonlinear k-space encoding is required if contrast-relevant k-space regions need to be acquired at specific delays following magnetization preparation or for transient artifact reduction in cardiac-gated k-space segmented CINE imaging. Such trajectories are problematic for balanced SSFP imaging due to nonconstant eddy current effects and resulting disruption of the steady state. In this work, a novel acquisition strategy for balanced SSFP imaging is presented that utilizes scan time reduction by parallel imaging for optimized "double average" eddy current compensation and artifact reduction during the transient phase following steady-state storage and magnetization preparation. Double average parallel SSFP imaging was applied to k-space segmented CINE SSFP tagging as well as nongated centrically encoded SSFP imaging. Phantom and human studies exhibit substantial reduction in steady-state storage and eddy current artifacts while maintaining spatial resolution, signal-to-noise ratio, and similar total scan time of a standard SSFP acquisition. The proposed technique can easily be extended to other acquisition schemes that would benefit from nonlinear reordering schemes and/or rely on interruption of the balanced SSFP steady state.

Algorithms↗

PubMed1993

Circadian rhythms in prokaryotes: luciferase as a reporter of circadian gene expression in cyanobacteria.

We have used a luciferase reporter gene and continuous automated monitoring of bioluminescence to demonstrate unequivocally that cyanobacteria exhibit circadian behaviors that are fundamentally the same as circadian rhythms of eukaryotes. We also show that these rhythms can be studied by molecular methods in Synechococcus sp. PCC7942, a strain for which genetic transformation is well established. A promoterless segment of the Vibrio harveyi luciferase structural genes (luxAB) was introduced downstream of the promoter for the Synechococcus psbAI gene, which encodes a photosystem II protein. This reporter construction was recombined into the Synechococcus chromosome, and bioluminescence was monitored under conditions of constant illumination following entrainment to light and dark cycles. The reporter strain, AMC149, expressed a rhythm of bioluminescence which satisfies the criteria of circadian rhythms: persistence in constant conditions, phase resetting by light/dark signals, and temperature compensation of the period. Rhythmic changes in levels of the native psbAI message following light/dark entrainment supported the reporter data. The behavior of this prokaryote disproves the dogma that circadian mechanisms must be based on eukaryotic cellular organization. Moreover, the cyanobacterial strain described here provides an efficient experimental system for molecular analysis of the circadian clock.

Circadian Rhythm↗