Dinoflagellate Bioluminescence

Introduction

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Figure 1. Electron micrograph of the Ebola Zaire virus. This was the first photo ever taken of the virus, on 10/13/1976. By Dr. F.A. Murphy, now at U.C. Davis, then at the CDC.[1].

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Legend/credit: Electron micrograph of the Ebola Zaire virus. This was the first photo ever taken of the virus, on 10/13/1976. By Dr. F.A. Murphy, now at U.C. Davis, then at the CDC.
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Section 1 Luciferin and Luciferase

Evolution and Genetics of Luciferase

The evolution of bioluminescence in dinoflagellates is largely due to the appearance of the dinoflagellate luciferase gene (lcf), which encodes the enzyme luciferase. Luciferase catalyzes the reaction that produces light. All bioluminescent dinoflagellates have the lcf gene, but there is a lot of variation between genera in the exact nucleotide sequence . One example of this variation is the number of active domains in luciferase, with some species having three while others only have one. At least in the species Gonyaulax polyedra, the gene is a sequence of tandem repeats that seem to have promoters in between.

The mechanism for transcription of the luciferase gene is unknown. Dinoflagellate chromosomes are unique because they do not have histones and are all always condensed, so the overall mechanism of transcription is likely different from that of other eukaryotes. <1> Regulation of the gene coding for luciferase is particularly interesting because its expression is regulated by a circadian rhythm, with increased expression at night when bioluminescence can actually be seen. Despite this interest, the mechanism is still unknown, but some circadian regulation is thought to be done post-transcriptionally.

Luciferin

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Section 2 Mechanism of Bioluminescence

Bioluminescence in dinoflagellates is triggered by physical agitation, which causes an influx of protons into specialized organelles called scintillons, which are where bioluminescence takes place. Scintillons will protrude into very acidic vacuoles, so when stress is applied a mechanotransduction pathway, where mechanical stimuli are converted to biochemical signals, is activated, causing an action potential along the vacuole and scintillon membranes. This action potential triggers the opening of voltage gated proton channels, allowing protons to flow from the acidic vacuole into the scintillons.

Dinoflagellates only produce light when a luciferase enzyme catalyzes the oxidation of a luciferin. However, in high pH environments the a-helices of luciferase block access of the luciferin substrate to the catalytic β-barrel where the oxidation occurs. However at a low pH, such as that caused by the influx of protons into the scintillons, the a-helices move out of the way, allowing the reaction to proceed. The exact mechanism of this is unproven, but the leading hypothesis suggests that it is due to the protonation of several histidines in the polypeptide, as well as a lysine residue. These protonations cause electrostatic repulsion between the histidine-cations, leading to the conformational change uncovering the active site in the β-barrel.

Once the active site is uncovered, luciferin is oxidized by luciferase. This causes the release of photons with a wavelength of about 475 nm, which is perceived by humans as a flash of blue light. How the actual production of light works, and why light is released instead of other types of energy, is not well understood.

A second form of regulation, possibly relating to the circadian rhythm which prevents luminescence during the day but encourages it during the night, is the luciferin binding protein (LBP). In some species, this protein binds to luciferin and only unbinds at low pH, creating a similar regulatory effect as described above. (cite)

Conclusion

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References

Edited by Dylan Ryznar, student of Joan Slonczewski for BIOL 116, 2024, Kenyon College.