PubMed Health⌕ Search

Biomedical subjects

W Marwan

Publications and source records attributed to W Marwan.

24 records · Page 2Linked to original sources
PubMed1990

Quantitation of photochromism of sensory rhodopsin-I by computerized tracking of Halobacterium halobium cells.

The swimming behavior of Halobacterium halobium is controlled by light which acts through retinal photoreceptor proteins. The sensing of near-ultraviolet (u.v.) was proposed to be mediated by the thermally metastable intermediate SR-I373 that is formed upon orange light absorption by sensory rhodopsin-I (SR-I). In order to test the validity of this proposal, we analyzed the photochromic behavior of the functional near-u.v. receptor in situ by use of an automated cell tracking system. The system was specifically designed for detection of swimming reversals in individual cells and calibrated with a straight-swimming mutant of H. halobium. Quantitative analysis of the response of the cells to near-u.v. revealed that orange background light increased the number of active near-u.v. receptor molecules. The intensity-dependence of this effect fitted into the kinetic scheme of a photochromic receptor pigment. The half-life of the functional near-u.v. receptor species was determined under continuous orange background light and found to be similar to that of the SR-I373 intermediate of sensory rhodopsin-I in intact cells. These results clearly support the assignment of the near-u.v. receptor to SR-I373. The kind of kinetic analysis described here, might be a useful tool in assigning spectroscopic data of pigments to photoreceptor function also in other organisms.

Bacteriorhodopsins↗
PubMed1990

Signal transduction in Halobacterium depends on fumarate.

The isolation of a straight-swimming mutant of Halobacterium halobium is reported which has a defect in switching the rotational sense of its flagellar motor. Cells of this mutant strain could be complemented with an extract from wild-type cells by mild sonication and resealing of the cells in fresh medium. The switch factor responsible for restoration of wild-type behaviour was isolated from membrane vesicle preparations. Its chemical nature is proposed to be that of fumarate on the basis of chemical, chromatographic and mass spectrometric analysis. Since the switch factor (fumarate) was released from a membrane-bound state by heat and was accumulated into mutant cells that lack this compound, it is proposed that a membrane-bound protein exists which specifically binds the switch factor. Both the switch factor and fumarate cause stimulus-induced responses in cells at the level of one or few molecules.

Cell Movement↗
PubMed1989

Biosynthesis of the two halobacterial light sensors P480 and sensory rhodopsin and variation in gain of their signal transduction chains.

The two retinal-containing photoreceptors of halobacteria, P480 and sensory rhodopsin, are formed constitutively and inducibly, respectively. Both photoreceptors are synthesized as apoproteins in cells with nicotine-inhibited retinal synthesis and are reconstituted as chromoproteins by the addition of all-trans retinal to cell membrane preparations. The decrease in photoreceptor-mediated photophobic response at the stationary growth phase of cells is not due to photoreceptor degradation but due to a deficiency of the signal transduction chain in the cell.

Bacterial Proteins↗
PubMed1987

Signal formation in the halobacterial photophobic response mediated by a fourth retinal protein (P480).

Halobacterial cells swim forward by clockwise, and backward by counterclockwise, rotation of their flagella. The changes of direction of rotation occur statistically and can be quantitatively described by a four-state model of the motor. Stimulation of the cells with blue light induces the formation of a signal that causes the motor to switch the direction of rotation. The results of step-up and flash experiments led to a kinetic equation that describes the signal formation as a photocatalytic process. The stimulating blue light is sensed either by sensory rhodopsin in the presence of green background light or by protein P480, which has a maximum in the action spectrum around 480 nm. P480, but not sensory rhodopsin, is synthesized by the cells constitutively, and both pigments together allow the cells to find optimal conditions during aerobic and phototrophic growth. The work presented here was reported at the U.S.-Israel Binational Science Foundation Meeting in Jerusalem, March 1986.

Dose-Response Relationship, Radiation↗
PubMed1987

Change of membrane potential is not a component of the photophobic transduction chain in Halobacterium halobium.

Long (20 to 50 microns) and bipolarly flagellated cells of Halobacterium halobium were stimulated locally by a focused beam of light, and the photophobic response was analyzed. The results demonstrate that two flagellar bundles did not react in a coordinated fashion. The light-induced stop response of a flagellar bundle only occurred if the stimulus was applied within 5 microns of the polar region. This excluded membrane potential changes from being causally involved in photophobic signalling and indicated that there is a diffusible messenger in the signal transduction chain which is subjected to decay. In addition, the photoreceptor may be localized at the polar end of the cell.

Cell Movement↗