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PubMedDate not supplied

A 221-bp fragment of the mouse opsin promoter directs expression specifically to the rod photoreceptors of transgenic mice.

Mutations in the human rod opsin gene have been shown to segregate with autosomal dominant retinitis pigmentosa (ADRP) and photoreceptor degeneration in transgenic mice. While these degenerations are characterized by the primary degeneration of rods, cones eventually die as well. To determine whether this subsequent cone degeneration is the result of expression of mutant rod opsin in the cones, the retinal cell-type specificity of a 221-bp fragment of the mouse rod opsin promoter was evaluated. Two transgenic mouse lines generated by injecting a fusion gene comprised of a 221-bp fragment of the mouse rod opsin promoter and the simian virus 40 large tumor antigen gene (Tag) were examined. The expression of Tag causes photoreceptor cell degeneration in members of both transgenic lines. However, the two lines differed with respect to the level of Tag expression and the rate and extent of photoreceptor cell degeneration. Immunocytochemical localization of opsin and Tag in surviving photoreceptor cells was determined and the results were confirmed by reverse transcriptase polymerase chain reaction (RT-PCR). Rod- and cone-mediated function was evaluated by electroretinography (ERG). In the higher Tag-expressing transgenic line only one row of nuclei remained in the outer nuclear layer at postnatal day (P) 150. While these nuclei showed no antigenicity for rod opsin or Tag, they did stain with an antibody that reacts with both rod and cone S-antigens (arrestins), indicating that these cells were surviving photoreceptor nuclei. Positive staining with peanut agglutinin, which uniquely decorates matrix domains surrounding cones in the normal retina, confirmed that the surviving photoreceptor nuclei were of cone origin. RT-PCR substantiated the results from immunostaining; amplification product was obtained using blue cone opsin transcripts but not from either Tag or rod opsin transcripts. The second transgenic mouse line exhibited a much slower photoreceptor cell death that was associated with low levels of Tag transgene transcript. At P120, approximately 50% of photoreceptors remained and an approximately 45% reduction in the rod ERG a-wave was observed. Cone-mediated ERGs, however, were normal. The results demonstrate the rod-specific expression of Tag as directed by the 221-bp fragment of the mouse rod opsin promoter and suggest that the cone degeneration in ADRP or transgenic mice associated with mutations in the rod opsin gene is a secondary effect of rod degeneration.

Animals↗
PubMed2000

A novel rod-like opsin isolated from the extra-retinal photoreceptors of teleost fish.

We have isolated a novel opsin from the pineal complex of Atlantic salmon (Salmo salar) and from the brain of the puffer fish (Fugu rubripes). These extra-retinal opsins share approximately 74% identity at the nucleotide and amino acid level with rod-opsins from the retina of these species. By PCR, we have determined that the novel rod-like opsin is not expressed in the salmon retina, and the retinal rod-opsin is not expressed in the salmon pineal. Phylogenetic analysis suggests that the rod-like opsins arose from a gene duplication event approximately 205 million years ago, a time of considerable adaptive radiation of the bony fish. In view of the large differences in the coding sequences of the pineal/brain rod-like opsins, their extra-retinal sites of expression, and phylogenetic position we have termed these novel opsins 'extra-retinal rod-like opsins' (ERrod-like opsins). We speculate that the differences between retinal rod-opsins and ERrod-like opsins have arisen from their differing photosensory roles and/or genetic drift after the gene duplication event in the Triassic.

Amino Acid Sequence↗
PubMed2004

Photoreceptor differentiation during retinal development, growth, and regeneration in a metamorphic vertebrate.

To test the hypothesis that growth and regeneration of the adult retina involves a mechanistic recapitulation of retinal development, the patterns of photoreceptor differentiation were investigated in the developing retina, as well as growing and regenerating adult retina, of a metamorphic vertebrate, the winter flounder. Only one opsin, of type RH2 (a "green" cone opsin), was expressed in premetamorphic (developing) retina, and a corresponding middle-wavelength visual pigment was observed. In premetamorphic retinas there was no evidence for any other cone opsins or pigments, rods, rod opsin expression, or rod visual pigment. In contrast, a rod opsin (RH1) and three cone opsins (SWS2, RH2, and LWS) were expressed in postmetamorphic (adult) retina, and these opsins were consistent with the observed repertoire of visual pigments. During postmetamorphic retinal growth and regeneration, cones were always produced before rods, but the different cone types were apparently produced simultaneously, suggesting that cone differentiation mechanisms might change after metamorphosis. The results support the hypothesis that photoreceptor differentiation during growth and regeneration of the adult retina involves a recapitulation of mechanisms that control the sequence of photoreceptor production during retinal development.

Amino Acid Sequence↗
PubMed1999

Occupancy of the chromophore binding site of opsin activates visual transduction in rod photoreceptors.

The retinal analogue beta-ionone was used to investigate possible physiological effects of the noncovalent interaction between rod opsin and its chromophore 11-cis retinal. Isolated salamander rod photoreceptors were exposed to bright light that bleached a significant fraction of their pigment, were allowed to recover to a steady state, and then were exposed to beta-ionone. Our experiments show that in bleach-adapted rods beta-ionone causes a decrease in light sensitivity and dark current and an acceleration of the dim flash photoresponse and the rate constants of guanylyl cyclase and cGMP phosphodiesterase. Together, these observations indicate that in bleach-adapted rods beta-ionone activates phototransduction in the dark. Control experiments showed no effect of beta-ionone in either fully dark-adapted or background light-adapted cells, indicating direct interaction of beta-ionone with the free opsin produced by bleaching. We speculate that beta-ionone binds specifically in the chromophore pocket of opsin to produce a complex that is more catalytically potent than free opsin alone. We hypothesize that a similar reaction may occur in the intact retina during pigment regeneration. We propose a model of rod pigment regeneration in which binding of 11-cis retinal to opsin leads to activation of the complex accompanied by a decrease in light sensitivity. The subsequent covalent attachment of retinal to opsin completely inactivates opsin and leads to the recovery of sensitivity. Our findings resolve the conflict between biochemical and physiological data concerning the effect of the occupancy of the chromophore binding site on the catalytic potency of opsin. We show that binding of beta-ionone to rod opsin produces effects opposite to its previously described effects on cone opsin. We propose that this distinction is due to a fundamental difference in the interaction of rod and cone opsins with retinal, which may have implications for the different physiology of the two types of photoreceptors.

Algorithms↗
PubMedDate not supplied

Molecular cloning and localization of rhodopsin kinase in the mammalian pineal.

Several retinal photoreceptor proteins involved in phototransduction have also been found in the mammalian pineal. This study demonstrates that rat and human pineals express protein kinases that are identical to the corresponding rod photoreceptor rhodopsin kinases. The deduced amino acid sequence of rat and human rhodopsin kinases have 84% sequence similarity to the earlier reported sequence of the bovine retinal enzyme, with complete conservation of the topological regions containing the position of the catalytic domain and sites of posttranslational modifications. Rat pineal also expresses rod opsin and putative blue cone opsin. Using immunocytochemistry, rod opsin and rhodopsin kinase were found to be co-localized in pinealocytes in the human tissue. These data demonstrate that the mammalian pineal contains light-sensitive opsins and a kinase involved in their inactivation. These findings correlate with an earlier report that neonatal rats show extraretinal light sensitivity, and suggest that a functional photoreceptive system may be present in the adult mammalian pineal.

Adult↗
PubMed1997

Expression of visual and nonvisual opsins in American chameleon.

We previously characterized five visual opsin genes of American chameleon (Anolis carolinensis). Here we report its nonvisual opsin gene orthologous to the chicken pineal gland-specific opsin (p-opsin) gene. In the pure-cone American chameleon retina, all visual opsins including rod opsin are expressed. In both pineal and parietal eye, three visual opsins as well as P-opsin are expressed. Although opsins are detected in the pineal glands of a wide variety of vertebrates, Southern analysis suggests that the P-opsin gene is used mainly by birds and reptiles.

Amino Acid Sequence↗
PubMedDate not supplied

Spatio-temporal characterization of retinal opsin gene expression during thyroid hormone-induced and natural development of rainbow trout.

The abundance and spatial distribution of retinal cone photoreceptors change during thyroid hormone (TH)-induced and natural development of rainbow trout (Oncorhynchus mykiss). These changes are thought to allow the fish to adapt to different photic environments throughout its life history. To date, the ontogeny of rainbow trout cone photoreceptors has been examined using physiological and morphological approaches. In this study, we extended these observations by measuring opsin gene expression in retinal quadrants during natural and TH-induced development. Gene expression during natural development was investigated in retinae from fish at both parr and smolt stages. The role of TH in modulating opsin gene expression was determined in TH-treated parr and control fish sampled after two, nine, and 22 days of treatment. Total RNA was isolated from each retinal quadrant and steady-state opsin mRNA levels were measured using reverse transcriptase real-time quantitative polymerase chain reaction (QPCR) analysis. Expression of ultraviolet-sensitive opsin (SWS1), rod opsin (RH1), middle wavelength-sensitive opsin (RH2), and long wavelength-sensitive opsin (LWS) transcripts vary spatially in the parr retina. Smolts, compared to parr, had downregulated SWS1 expression in all quadrants, lower LWS expression dorsally, higher RH1 expression nasally, and higher RH2 expression dorsally. In TH-treated parr, SWS1 opsin expression was downregulated in the nasal quadrants by two days. SWS1 displayed the greatest degree of downregulation in all quadrants after nine days of treatment, with an increase in short wavelength-sensitive (SWS2) and RH2 opsin mRNA expression in the temporal quadrants. This study reveals that opsin genes display spatially significant differences within rainbow trout retina in their level of mRNA expression, and that regulation of opsin expression is a dynamic process that is influenced by TH. This is particularly evident for SWS1 gene expression in parr following TH-induced and natural development.

Animals↗
PubMed1992

Expression of developmentally defined retinal phenotypes in the histogenesis of retinoblastoma.

Retinoblastoma, the most common intraocular tumor of childhood, is a malignant neoplasm that arises during retinal development. The embryonal cell target for neoplastic transformation is not yet clearly defined. To better understand the histogenetic potential of this tumor, the expression of photoreceptor and glial cell-associated proteins were examined in 22 primary retinoblastomas. Interphotoreceptor retinol-binding protein (IRBP), cone and rod opsins were selected as the photoreceptor specific proteins due to their different temporal patterns of expression during normal retinal development. Neoplastic Müller cell differentiation, and non-neoplastic reactive astrocytes were identified using cellular retinaldehyde binding-protein (CRAlBP), and glial fibrillary acidic protein (GFAP), respectively. Photoreceptor proteins were present in 16 cases and showed different cellular patterns of expression. IRBP and cone opsin were usually abundant. Although rod opsin was clearly identified in eight tumors, its expression was more restricted than either IRBP or cone opsin. This differential pattern of expression, opposite to the normal pattern of photoreceptor gene expression in the adult retina, corresponded to a marked decrease in mRNA for rod opsin. Cone opsin and IRBP colocalized in fleurettes demonstrating that neoplastic human cone cells are capable of IRBP synthesis. Müller cell differentiation was present in 12 of the 16 cases in which photoreceptor proteins were detected. In contrast, GFAP was only present in reactive, stromal astrocytes associated with blood vessels. Our data suggest that the retinoblastoma has the histogenetic potential of the immature neural retinal epithelium which can give rise to both photoreceptor and Müller cell lineages. The differential expression of cone and rod phenotypes in retinoblastoma is consistent with the "default" mechanism of cone cell differentiation.

Blotting, Northern↗
PubMed2005

Sp4 is expressed in retinal neurons, activates transcription of photoreceptor-specific genes, and synergizes with Crx.

To investigate the molecular mechanisms of photoreceptor-specific gene transcription, we examined the role of the neuronal-enriched Sp4 nuclear protein in transcription from the rod-specific beta-PDE and rod opsin gene promoters and compared it to the ubiquitous members of the Sp family, Sp1 and Sp3. Sp4 activates both the rod opsin and beta-PDE promoters, whereas Sp1 activates only the rod opsin promoter and Sp3 activates neither promoter. Interestingly, Sp1 and Sp3 competitively repress Sp4-mediated activation of the beta-PDE promoter. In addition, Sp4, Sp1, and Sp3 each show functional synergy with the photoreceptor-enriched Crx transcriptional regulator on the rod opsin promoter but not the beta-PDE promoter, although Sp4-mediated activation was the most significant. Sp4, Sp1, and Sp3 bind Crx in co-immunoprecipitation experiments, and their zinc finger domains as well as the Crx homedomain are necessary and sufficient for these interactions. Chromatin immunoprecipitation showed that the rod opsin and beta-PDE promoters are targets of both Sp4 and Crx, which further supports Sp4-Crx interactions in vivo in the context of retinal chromatin environment. In situ hybridization and immunohistochemistry demonstrated that Sp4 is abundantly expressed in various neurons of all retinal layers, and thus co-localizes or overlaps with multiple retina-restricted and -enriched genes, its putative targets. Our results indicate that photoreceptor-specific gene transcription is controlled by the combinatorial action of Sp4 and Crx. The other Sp family members may be involved in photoreceptor-specific transcription directly or through their competition with Sp4. These data suggest the potential importance of Sp4 in retinal neurobiology and pathology.

Animals↗
PubMed1996

The spatio-temporal pattern of photoreceptor degeneration in the aged rd/rd mouse retina.

Photoreceptor degeneration in the retina of the rd/rd (retinal degeneration) mice has been studied using immunocytochemistry with antisera against cone- and rod-opsin. The rd/rd mice exhibited different regional specific rates of degeneration for rods and cones. As early as postnatal day 25, cells labelled with the rod-opsin and cone-opsin antisera disappeared preferently from the central retina. Whereas in the inferior half of the retina, degeneration subsequently proceeded towards the periphery, this did not occur in the dorsal hemisphere. By the age of 100 days, many cells immunoreactive for the cone-opsin antiserum and a few cells immunoreactive for the rod-opsin antiserum were located in an area of the dorsal retina. The ventral retina lacked labelled elements at this age. Finally, rd/rd mice at one year or 600 days of age contained a similar number of cone-opsin immunopositive cells (approximately 2000-2800 cells), occupying almost the same area in the retina as that found at 100 days of age. A photoreceptor candidate for the entrainment of non-visual photoreception probably remains in the cone population in aged rd/rd mice.

Aging↗
PubMed1998

Novel expression pattern of interphotoreceptor retinoid-binding protein (IRBP) in the adult and developing zebrafish retina and RPE.

PURPOSE: Interactions between the neural retina and retinal pigment epithelium (RPE) are mediated by the interphotoreceptor matrix (IPM). The transport of retinoids across the IPM is mediated by interphotoreceptor retinoid-binding protein (IRBP). To explore the possibility that IRBP is important during retinal development, we examined its spatiotemporal expression pattern in embryonic zebrafish. METHODS: IRBP mRNA expression was examined using RT-PCR and in situ hybridization. IRBP was localized using antiserum against recombinant zebrafish IRBP. IRBP synthesis and secretion were studied by in vitro metabolic labeling of retinas and RPE-eyecups. RESULTS: IRBP mRNA was first observed in the pineal at 24 hours post-fertilization (hpf) and in the ventral retina at 50 hpf. Immunoreactive IRBP was first observed at 72 hpf. Remarkably, IRBP was expressed not only by photoreceptors but also by the adult and embryonic RPE. In embryos, expression in both retina and RPE began in a ventronasal patch and spread to involve the entire eye. In general, early IRBP expression was dominated by photoreceptors, but then RPE expression spread beyond the limit of photoreceptor expression. Double in situ hybridizations suggests that cones express IRBP mRNA before they express a specific opsin, while rods may express rod opsin prior to IRBP. CONCLUSIONS: The temporal and spatial patterns of IRBP expression by the RPE and retina are consistent with a role in retinal development and suggest coordination of RPE and photoreceptor differentiation.

Animals↗
PubMed1991

Photoreceptor differentiation in cerebellar medulloblastoma: evidence for a functional photopigment and authentic S-antigen (arrestin).

The aim of the present study was to evaluate the putative photoreceptor differentiation found in certain cerebellar medulloblastomas. The analyses were focussed on S-antigen, rod-opsin (the apoprotein of the visual pigment rhodopsin) and 11-cis retinal (the prosthetic group of rhodopsin). Fresh frozen and paraffin-embedded biopsy specimens of three medulloblastomas were investigated by means of immunocytochemistry, enzyme-linked immunosorbent assay (ELISA), high-pressure liquid chromatography (HPLC), and immunoblotting. As shown in paraffin sections, one out of the three tumors (tumor A) contained S-antigen- and rod-opsin-immunoreactive tumor cells. The immunoblotting technique revealed in this tumor a single protein band of approximately 48-50 kDa that reacted with the S-antigen antibody and three protein bands of approximately 40, 75 and 110 kDa recognized by the rod-opsin antibody. These bands could not be detected in the two remaining tumors (tumor B and C). The rod-opsin content of tumor A was quantified by the ELISA; 11.7 pmol rod-opsin were calculated for the biopsy. The HPLC demonstrated the presence of 11-cis- and all-trans-retinal in tumor A, but not in tumors B and C. Furthermore, it was shown that 11-cis-retinal was converted to all-trans-retinal upon illumination of the tumor extract. The ratio between 11-cis- and all-trans-retinal was approximately 1:1 before illumination and 3:5 after illumination. A total of 2-3 pmol of retinal was found in the biopsy of tumor A. In addition all-trans-retinol was present in this tumor. The results indicate that certain medulloblastomas express a functional photopigment and S-antigen, another protein of the phototransduction cascade. They strongly support the concept that medulloblastoma cells may differentiate along the photoreceptor cell lineage.

Antigens↗
PubMed1995

Immunocytochemical localization of opsin in rod photoreceptors during periods of rapid disc assembly.

Transport of opsin from photoreceptor inner to outer segments has been assumed to occur via the connecting cilium, the only permanent structural connection between these two regions. However, in prior work, little or no immunoreactive opsin has been detected in the cilium, despite the high rate of transport of this protein. This suggests that immune epitopes are masked during passage through the cilium or that opsin is transported via an extra-ciliary route. In this study, we stained the photoreceptors of Xenopus laevis with well-characterized monoclonal antibodies directed at the N-terminal, C-terminal, and 5-6 loop regions of bovine opsin. This was done on isolated retinas incubated in vitro under conditions that support rapid disc assembly, to insure that opsin transport to forming discs was occurring at the time of fixation. Five MAbs that gave robust staining of Xenopus rod inner segment/rod outer segment preparations with the light microscope were utilized for electron microscopic studies on LR White embedded or cryo-ultrathin sections. Four of these stained outer segment discs and inner segment vesicles and plasma membrane. However, no significant staining of the connecting cilium was found. Furthermore, freeze-fractured mouse photoreceptors prepared by the 'fracture-label' technique showed extensive labelling of membrane compartments but lacked staining of the connecting cilium. Isolated retinas incubated under conditions that support robust rod disc synthesis contained many finger-like and vesicular projections of the apical inner segment plasma membrane and inner segment vesicles extending into them. Rod outer segment nascent discs usually made close contact with the inner segment. Both the vesicular profiles associated with the inner segment plasma membrane and the basal discs extending to the inner segment were heavily stained with all four anti-opsin antibodies. This suggests an alternate route for bulk transport of opsin to newly forming discs that involves direct transfer from apical inner segment plasma membrane to nascent discs.

Animals↗
PubMed1994

Relationships of G-protein-coupled receptors. A survey with the photoreceptor opsin subfamily.

Relationships among the G-protein-coupled receptors were evaluated using several distance matrices with the neighbor-joining method of Saitou and Nei (1987). The relationships generated vary depending upon alignment, length, or region of sequence compared, and the distance matrix used to score similarity. To provide a statistical level of confidence, bootstrap resampling was applied to the analysis of a selection of G-protein-coupled receptors and the subfamily photoreceptor opsins. A general consensus indicates that the opsins behave as a discrete subfamily among the superfamily of G-protein-coupled receptors. Their relationship to other subfamilies remains unresolved. Within the opsin subfamily, the retinochromelike opsins segregate as a discrete group, but are more closely related to the invertebrate than vertebrate opsins. Among vertebrate opsins, the long wavelength cone opsins, the blue/violet opsins, and the rod opsins (including a class of green cone opsins) form distinct subgroups, but their relationships to one another remain unresolved. For this superfamily of receptors, the confidence levels for many branch pairings are low. The application of methods complimentary to those used in this preliminary study will be necessary to resolve questions about appropriate pairing and evolutionary relationships.

Amino Acid Sequence↗
PubMed2004

Novel dominant rhodopsin mutation triggers two mechanisms of retinal degeneration and photoreceptor desensitization.

A variety of rod opsin mutations result in autosomal dominant retinitis pigmentosa and congenital night blindness in humans. One subset of these mutations encodes constitutively active forms of the rod opsin protein. Some of these dominant rod opsin mutant proteins, which desensitize transgenic Xenopus rods, provide an animal model for congenital night blindness. In a genetic screen to identify retinal degeneration mutants in Drosophila, we identified a dominant mutation in the ninaE gene (NinaE(pp100)) that encodes the rhodopsin that is expressed in photoreceptors R1-R6. Deep pseudopupil analysis and histology showed that the degeneration was attributable to a light-independent apoptosis. Whole-cell recordings revealed that the NinaE(pp100) mutant photoreceptor cells were strongly desensitized, which partially masked their constitutive activity. This desensitization primarily resulted from both the persistent binding of arrestin (ARR2) to the NINAE(pp100) mutant opsin and the constitutive activity of the phototransduction cascade. Whereas mutations in several Drosophila genes other than ninaE were shown to induce photoreceptor cell apoptosis by stabilizing a rhodopsin-arrestin complex, NinaE(pp100) represented the first rhodopsin mutation that stabilized this protein complex. Additionally, the NinaE(pp100) mutation led to elevated levels of G(q)alpha in the cytosol, which mediated a novel retinal degeneration pathway. Eliminating both G(q)alpha and arrestin completely rescued the NinaE(pp100)-dependent photoreceptor cell death, which indicated that the degeneration is entirely dependent on both G(q)alpha and arrestin. Such a combination of multiple pathological pathways resulting from a single mutation may underlie several dominant retinal diseases in humans.

Animals↗
PubMed1999

Glial cell line-derived neurotrophic factor induces histologic and functional protection of rod photoreceptors in the rd/rd mouse.

PURPOSE: To evaluate the neuroprotective potential of glial cell line-derived neurotrophic factor (GDNF) in the retinal degeneration (rd/rd) mouse model of human retinitis pigmentosa. METHODS: Subretinal injections of GDNF were made into rd/rd mice at 13 and 17 days of age and electroretinograms (ERGs) recorded at 22 days. Control mice received saline vehicle injections or underwent no procedure. At 23 days of age, retinas from treated and control mice were fixed and processed for wholemount immunohistochemistry using an anti-rod opsin antibody, and rod numbers were estimated using an unbiased stereological systematic random approach. Subsequent to counting, immunolabeled retinas were re-embedded and sectioned in a transverse plane and the numbers of rods recalculated. RESULTS: Although ERGs could not be recorded from sham-operation or nonsurgical rd/rd mice at 22 days of age, detectable responses (both a- and b-waves) were observed in 4 of 10 GDNF-treated mice. Stereological assessment of immunolabeled rods at 23 days showed that control rd/rd retinas contained 41,880+/-3,890 (mean +/- SEM; n = 6), phosphate-buffered saline (PBS)-injected retinas contained 61,165+/-4,932 (n = 10; P < 0.001 versus control retinas) and GDNF-injected retinas contained 89,232+/-8,033 (n = 10; P < 0.001 versus control retinas, P < 0.002 versus PBS). This increase in rod numbers after GDNF treatment was confirmed by cell counts obtained from frozen sections. CONCLUSIONS: GDNF exerts both histologic and functional neuroprotective effects on rod photoreceptors in the rd/rd mouse. Thus rescue was demonstrated in an animal model of inherited retinal degeneration in which the gene defect was located within the rods themselves, similar to most forms of human retinitis pigmentosa. GDNF represents a candidate neurotrophic factor for palliating some forms of hereditary human blindness.

Animals↗
PubMed1971

Energy transfer in rhodopsin, N-retinyl-opsin, and rod outer segments.

N-retinyl, the chromophore of bleached and reduced rhodopsin, N-retinyl-opsin, was used as a covalently attached fluorescence probe to examine the structure of N-retinyl-opsin and the rod outer segment. The efficiency of energy transfer from the protein part of N-retinyl-opsin to the chromophore is 12 +/- 5%. It is argued that this implies that the N-retinyl-opsin molecule is asymmetrical. Kropf has estimated the efficiency of energy transfer from the protein to the chromophore in native rhodopsin to be about 50%. This difference of efficiencies seems to imply a large movement of the chromophore away from the tryptophans of the opsin after rhodopsin is bleached. From excitation spectrum measurements, it has been found that light absorbed by the protein of the rod outer segments has more action in sensitizing the fluorescence of the chromophore than does light absorbed by the protein part of pure N-retinyl-opsin. Thus, some other tryptophans or tyrosines in either another N-retinyl-opsin molecule or another protein must be close enough (about 28 A) to the chromophore to transfer energy to it. Measurements of the polarization of the fluorescence of the chromophore suggest, however, that the chromophores of neighboring N-retinyl-opsin molecules are more than 20 A apart. Moreover, these neighboring chromophores do not transfer energy to each other, tending to rule out any clustering of chromophores of different N-retinyl-opsin molecules and suggesting that rhodopsin chromophores do not transfer energy to each other.

Energy Transfer↗