Squid, and a variety of fish, have luminescent
bacteria housed in special sacs called light organs. The bacteria
generate light with an enzyme called luciferase - but not all the
time.
When the bacteria Vibrio fischeri swim free in sea water, their
population densities are very low and they do not produce light. When they inhabit the
light organs of squid, however, the squid cells nurture the bacteria and their populations
soar to high levels. When the bacteria are packed tight, they begin to secrete messenger
proteins called autoinducer. As this reaches a critical level, it enters the bacteria and
activates a protein called LuxR.
Activated LuxR stimulates the bacterial
genes to produce the light producing proteins luciferin, luciferase and
associated light producing proteins.
The genes are stimulated to produce additional autoinducer
messengers and these are released from the bacteria. The additional production of
autoinducer messengers, triggered by the active LuxR, stimulate another round of protein
synthesis. This positive feedback loop results in rapid production of light producing
proteins. When these proteins reach a critical density, they react with each other to
liberate that delightfully eerie blue phosphorescence of the sea.
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