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At least 1,603 records · Page 89Linked to original sources
PubMed2002

Co-ordinated expression of phycobiliprotein operons in the chromatically adapting cyanobacterium Calothrix PCC 7601: a role for RcaD and RcaG.

In the cyanobacterium Calothrix sp. PCC 7601 the cpc2 operon encoding phycocyanin 2 (PC2) is expressed if red radiations are available. RcaD was previously identified in extracts from red-light-grown cells as an alkaline phosphatase-sensitive protein that binds upstream of the transcription start point (TSP) of the cpc2 operon. In this work, RcaD was purified, and the corresponding gene cloned with a PCR probe obtained using degenerated primers based on RcaD peptide sequences (accession no. AJ319541). Purified RcaD binds to the cpc2 promoter region and also to those of the constitutive cpc1 and apc1 operons that encode phycocyanin 1 and allophycocyanin. Escherichia coli-overexpressed RcaD can bind to the cpc2 promoter region. The rcaD gene is upstream of an open reading frame (ORF) termed rcaG. Co-transcription of both genes was demonstrated by reverse transcription (RT)-PCR experiments, and found to be independent of the light wavelengths. A single TSP was mapped. Sequence features of RcaD and RcaG led us to propose a functional relationship between these two proteins. A rcaD mutant generated by allelic exchange exhibited altered expression of the cpc2, cpeBA, apc1 and cpc1 operons upon green to red-light shifts. RcaD seems to be a co-activator co-ordinating the transcription of the phycobiliprotein operons upon changes in light spectral quality.

Adaptation, Physiological↗
PubMed2001

[Effect of chromatic flicker on circulation of the optic nerve].

PURPOSE: To determine the response of human optic nerve head (ONH) blood flow (delta F) to heterochromatic equiluminant flicker modulation (eql-fl) and compare it to the response induced by pure luminance flicker (l-fl). METHODS: In 5 normal volunteers the ONH blood flow was measured by conventional laser Doppler flowmetry. Stimuli were generated by green and red light emitting diodes and delivered to the eye through a fundus camera illumination optic. Both green and red illuminances were square wave modulated in counter phase at different frequencies between 2 and 40 Hz. delta F was defined as the ratio between the ONH blood flow after 1 min stimulation and a baseline blood flow measured prior to the stimulation. RESULTS: In response to a 2 Hz eql-fl, ONH blood flow increases by 36% in average. delta F versus flicker frequency displayed the characteristics of a low-pass function with a cutoff frequency of 10 Hz for an eql-fl and a band-pass function with broad maximum around 10 Hz for the I-fl. CONCLUSIONS: delta F in human ONH can be evoked by heterochromatic equiluminant flicker modulation. The blood flow frequency response to eql-fl and I-fl are similar to the neural activity dominated by the Parvo- and Magno-cellular activity, respectively. These findings offer a new approach to study the neurovascular coupling at the ONH in both physiological and diseased conditions involving predominantly or selectively the Magno- and Parvo-pathways.

Adult↗
PubMed2000

Role of chromaticity, contrast, and local orientation cues in the perception of density.

We compared the role of the red-green, blue-yellow, and luminance post-receptoral mechanisms in the perception of density. The task requires the comparison of densities between two stimuli composed of oriented bandpass elements, pseudo-randomly scattered across an area of constant size. The perception of density differences was measured by a temporal 2AFC procedure for all pairs of mechanisms and for four possible densities. We found that stimuli of identical physical densities are not perceived equally: there is a consistent bias in favour of blue-yellow stimuli which are perceived as significantly more dense than red-green and achromatic stimuli. We considered three factors that could have differentially affected the density perception of blue-yellow stimuli: an increase in the perceived size of the individual blue-yellow elements, a perceived contrast difference, and the presence of local orientation cues. We found that the increased perceived density of the blue-yellow stimuli occurred despite the fact that there was no increase in perceived size of the individual elements, and remained despite corrections for the two other factors. We conclude that the significant increase in perceived density for the blue-yellow mechanism is a global effect, associated with a perceived colour 'melting' of the elements in the array. Our data were fitted with the occupancy model of Allik and Tuulmets (1991, Perception & Psychophysics 49 303-314) and we found that blue-yellow stimuli have a greater 'occupancy' than red-green or achromatic stimuli.

Color Perception↗
PubMed1999

Perceptual motion standstill in rapidly moving chromatic displays.

In motion standstill, a quickly moving object appears to stand still, and its details are clearly visible. It is proposed that motion standstill can occur when the spatiotemporal resolution of the shape and color systems exceeds that of the motion systems. For moving red-green gratings, the first- and second-order motion systems fail when the grating is isoluminant. The third-order motion system fails when the green/red saturation ratio produces isosalience (equal distinctiveness of red and green). When a variety of high-contrast red-green gratings, with different spatial frequencies and speeds, were made isoluminant and isosalient, the perception of motion standstill was so complete that motion direction judgments were at chance levels. Speed ratings also indicated that, within a narrow range of luminance contrasts and green/red saturation ratios, moving stimuli were perceived as absolutely motionless. The results provide further evidence that isoluminant color motion is perceived only by the third-order motion system, and they have profound implications for the nature of shape and color perception.

Color Perception↗
PubMed2004

Chromate reduction at low sulphate concentration in hydrogen-fed bioreactors.

Aimed at developing a bioremediation process to treat Cr(VI)-bearing water at low sulphate concentration in order to reduce excess sulphide production, the highly toxic, mutagenic, and soluble Cr(VI) was reduced to the less toxic and insoluble Cr(III) in 2-litre fixed-bed reactors inoculated with the sulphate-reducing bacterium (SRB) Desulfomicrobium norvegicum, capable of performing direct enzymatic Cr(VI) reduction. H2 was used as the electron source. The fixed-films were developed on three different supports: a PVC cross-flow material, a pozzolana, and a ceramic granulate. The phased experiments began with a progressive increase of the Cr(VI) concentration in the feed to the column reactors, followed by a progressive decrease of the sulphate concentration. Inhibition by Cr(VI) was less pronounced with pozzolana than with the other supports; when the pozzolana column was fed with a medium containing 100 mg l(-1) Cr(VI) and only 250 mg l(-1) sulphate, the lowest residence time that could be applied for complete Cr(VI) reduction was 16 h. The molar ratio between the sulphate and Cr(VI) reduction rates was decreased down to 1.5, suggesting that indirect reaction with HS was not the sole mechanism of Cr(VI) reduction.

Bioreactors↗
PubMed1973

Complementary chromatic adaptation in a filamentous blue-green alga.

Fluorescent and red light environments generate greatly different patterns of pigmentation and morphology in Fremyella diplosiphon. Most strikingly, red-illuminated cultures contain no measurable C-phycoerythrin and have a mean filament length about 10 times shorter than fluorescent-illuminated cultures. C-phycoerythrin behaves as a photoinducible constituent of this alga. Spectrophotometric and immunochemical procedures were devised so that C-phycoerythrin metabolism could be studied quantitatively with [(14)C]-phenylalanine pulse-chased cultures. Transfer of red-illuminated cultures to fluorescent light initiates C-phycoerythrin production by essentially de novo synthesis. C-phycoerythrin is not degraded to any significant extent in cultures continuously illuminated with fluorescent light. Transfer of fluorescent-illuminated cultures to red light causes an abrupt cessation of C-phycoerythrin synthesis. The C-phycoerythrin content of cultures adapting to red light decreases and subsequently becomes constant. Loss of C-phycoerythrin is not brought about by metabolic degradation, but rather by a decrease in mean filament length which is effected by transcellular breakage. In this experimental system, light influences intracellular C-phycoerythrin levels by regulating the rate of synthesis of the chromoprotein.

Adaptation, Physiological↗
PubMed1981

A new class of visual defect. Spreading inhibition elicited by chromatic light stimuli.

This paper deals with a single male subject, M., who has a well-defined visual defect of a kind not previously described. The defect is associated with a potent inhibitory response to long-wavelength stimuli, which spreads for up to 12 deg from the area of stimulation, suppressing M.'s detection of other high contrast stimuli. The effect can also be observed, weakly, with certain other coloured stimuli, but not with yellows or whites. Detailed psychophysical data are given for threshold sensitivity, colour matching, two-colour increment thresholds and visual acuity, all of which are more or less abnormal, depending on the stimulus colour. No abnormal effects were, however, observed with white light stimuli. On the basis of M.'s ability to fuse red and green random dot stereogram pairs, we argue that the inhibition arises centrally in the visual pathways. and we examine the implications of M.'s response characteristics for the analysis of central visual processing.

Adult↗
PubMed1987

Chromaticity and achromaticity. Evidence for a functional differentiation in visual field defects.

In the visual field defects of 10 patients who had suffered lesions in the postgeniculate part of the primary visual projection, red-green discrimination and achromatic target detection was tested. In addition, 8 of these patients were tested for detection of red and green targets. Targets were presented on a low photopic achromatic background, so that the red and green targets differed from the background both in intensity and in wavelength, whereas the achromatic target differed in intensity only. Six patients showed evidence of discriminating between red and green targets, 5 patients could also detect the colour targets, but none could detect the achromatic one that was presented at the same retinal position. These results imply that wavelength and intensity information are treated differentially, and suggest that these patients possess residual colour-opponent channels that subserve the defective part of the visual field.

Color Perception↗
PubMed2006

Ascorbate acts as a highly potent inducer of chromate mutagenesis and clastogenesis: linkage to DNA breaks in G2 phase by mismatch repair.

Here we examined the role of cellular vitamin C in genotoxicity of carcinogenic chromium(VI) that requires reduction to induce DNA damage. In the presence of ascorbate (Asc), low 0.2-2 microM doses of Cr(VI) caused 10-15 times more chromosomal breakage in primary human bronchial epithelial cells or lung fibroblasts. DNA double-strand breaks (DSB) were preferentially generated in G2 phase as detected by colocalization of H2AX and 53BP1 foci in cyclin B1-expressing cells. Asc dramatically increased the formation of centromere-negative micronuclei, demonstrating that induced DSB were inefficiently repaired. DSB in G2 cells were caused by aberrant mismatch repair of Cr damage in replicated DNA, as DNA polymerase inhibitor aphidicolin and silencing of MSH2 or MLH1 by shRNA suppressed induction of H2AX and micronuclei. Cr(VI) was also up to 10 times more mutagenic in cells containing Asc. Increasing Asc concentrations generated progressively more mutations and DSB, revealing the genotoxic potential of otherwise nontoxic Cr(VI) doses. Asc amplified genotoxicity of Cr(VI) by altering the spectrum of DNA damage, as total Cr-DNA binding was unchanged and post-Cr loading of Asc exhibited no effects. Collectively, these studies demonstrated that Asc-dependent metabolism is the main source of genotoxic and mutagenic damage in Cr(VI)-exposed cells.

Ascorbic Acid↗