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http://www.biomedcentral.com/1741-7007/10/107/abstract

Genomic organization, evolution, and expression of photoprotein and opsin genes in Mnemiopsis leidyi: a new view of ctenophore photocytes

BMC Biology 2012, 10:107 doi:10.1186/1741-7007-10-107

Published: 21 December 2012

Abstract

Background

Calcium-activated photoproteins are luciferase variants found in photocyte cells of bioluminescent jellyfish (Phylum Cnidaria) and comb jellies (Phylum Ctenophora). The complete genomic sequence from the ctenophore Mnemiopsis leidyi, a representative of the earliest branch of animals that emit light, provided an opportunity to examine the genome of an organism that uses this class of luciferase for bioluminescence and to look for genes involved in light reception. To determine when photoprotein genes first arose, we examined the genomic sequence from other early-branching taxa. We combined our genomic survey with gene trees, developmental expression patterns, and functional protein assays of photoproteins and opsins to provide a comprehensive view of light production and light reception in Mnemiopsis.

Results

The Mnemiopsis genome has 10 full-length photoprotein genes situated within two genomic clusters with high sequence conservation that are maintained due to strong purifying selection and concerted evolution. Photoprotein-like genes were also identified in the genomes of the non-luminescent sponge Amphimedon queenslandica and the non-luminescent cnidarianNematostella vectensis, and phylogenomic analysis demonstrated that photoprotein genes arose at the base of all animals. Photoprotein gene expression in Mnemiopsis embryos begins during gastrulation in migrating precursors to photocytes and persists throughout development in the canals where photocytes reside. We identified three putative opsin genes in theMnemiopsis genome and show that they do not group with well-known bilaterian opsin subfamilies. Interestingly, photoprotein transcripts are co-expressed with two of the putative opsins in developing photocytes. Opsin expression is also seen in the apical sensory organ. We present evidence that one opsin functions as a photopigment in vitro, absorbing light at wavelengths that overlap with peak photoprotein light emission, raising the hypothesis that light production and light reception may be functionally connected in ctenophore photocytes. We also present genomic evidence of a complete ciliary phototransduction cascade inMnemiopsis.

Conclusions

This study elucidates the genomic organization, evolutionary history, and developmental expression of photoprotein and opsin genes in the ctenophore Mnemiopsis leidyi, introduces a novel dual role for ctenophore photocytes in both bioluminescence and phototransduction, and raises the possibility that light production and light reception are linked in this early-branching non-bilaterian animal.

Keywords:

Bioluminescence; ctenophore; Mnemiopsis leidyi; opsin; photocyte; photoprotein; photoreception; phototransduction

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Bioluminescense Female glowworm

Bioluminescense Female glowworm

Photoproteins refer to bioluminescent proteins from luminous organisms. These proteins do not exhibit a luciferinluciferase reaction,[1] that is, a normal enzymesubstrate reaction. Instead, these proteins display luminescence proportional to the amount of the protein. Such proteins are stable as a luciferin-photoprotein complex, often until the addition of another required factor, such as Ca2+ for the photoprotein aequorin.

The term was first used to describe the unusual chemistry of the Chaetopterus luminescent system

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Bioluminescence is the production and emission of light by a living organism. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms and terrestrial invertebrates. Some Symbiotic organisms carried within larger organisms produce light.

Overview [edit]

Further information: List of bioluminescent organisms

Bioluminescence is a form of luminescence, or “cold light” emission by living organisms; less than 20% of the light generates thermal radiation. It should not be confused with iridescencestructural colorationphosphorescence.

By etymology, bioluminescence is a hybrid word, originating from the Greek bios for “living” and the Latin lumen “light”.

Bioluminescence is a form of chemiluminescence where light energy is released by a chemical reaction. Firefliesanglerfish, and other creatures produce the chemicals luciferin (a pigment) andluciferase (an enzyme).[4] The luciferin reacts with oxygen to create light. The luciferase acts as a catalyst to speed up the reaction, which is sometimes mediated by cofactors such as calciumions or ATP. The chemical reaction can occur either inside or outside the cell. In bacteria, the expression of genes related to bioluminescence is controlled by an operon called the Lux operon.[5]

Distribution [edit]

Bioluminescence occurs widely among some groups of animals, especially in the open sea; in some fungi and bacteria; and in various terrestrial invertebrates including insects. Many, perhaps most deep-sea animals produce light. Most marine light-emission is in the blue and green light spectrum, the wavelengths that pass furthest through seawater. However, some loose-jawed fishemit red and infrared light, and the genus Tomopteris emits yellow light. Sometimes thousands of square miles of the ocean shine with the light of bioluminescent bacteria in the “Milky seas effect“.[6]

Non-marine bioluminescence is less widely distributed. The two best-known forms of land bioluminescence are fireflies and glow worms. Other insects, insect larvaeannelidsarachnids and even species of fungi have been noted to possess bioluminescent abilities. Some forms of bioluminescence are brighter (or exist only) at night, following a circadian rhythm.

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Luciferins (from the Latin lucifer, “light-bringer”) are a class of light-emitting heterocyclic compounds found in organisms that cause bioluminescence. Luciferins typically undergo an enzyme-catalysed oxidation and the resulting unstable reaction intermediate emits light upon decaying to its ground state. The term luciferin is also used generically to refer both to the molecules that react with luciferases to emit light and to photoprotein, which emits light without the intervention of an enzyme

Luciferins are a class of small-molecule substrates that are oxidized in the presence of the enzyme luciferase to produce oxyluciferin and energy in the form of light. It is not known just how many types of luciferins there are, but some of the better-studied compounds are listed below. There are many types of luciferins, yet all share the use of reactive oxygen species to emit light.

File:Firefly luciferin.svg

Firefly_luciferin.svg

Nota: Porque aqui a F3 é substituida por S (enxofre) e a F6 é substituida por N ? E porque isto produz luz?

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