Microbiology: A Systems Approach 4th Edition by Marjorie Kelly Cowan

Exercise 26

Bioluminescence: The Oceans Are Glowing
Maybe you've been walking along a lovely beach at night when you noticed flashes of light coming from the waves. Or you might have noticed it in the wake of a ship in the night-iridescent blue water rippling around the hull. The glow has even been seen from space: In 1995, satellites detected luminescent water off the coast of Somalia for three consecutive nights. Sailors have described these "milky seas" for hundreds of years-there was even mention of a "milk sea" in Jules Verne's classic 20,000 Leagues Under the Sea. What causes these shimmering displays Dinoflagellates, a group of eukaryotic marine plankton, cause some of the iridescent surf, but in many cases, bacteria are the source of light in the oceans. The phenomenon of living organisms giving off light is called bioluminescence.
Many species of animals, fish, insects, and bacteria are bioluminescent. Scientists have studied these organisms and have discovered that certain species of bioluminescent bacteria live symbiotically with organisms that use the light they emit to attract prey or to scare off predators. Vibrio, Photorhabdus, and Photobacterium are some of the genera that live symbiotically with certain crustaceans and fish. Some of these organisms are free-living as well and cause the beautiful oceanic light displays we have discussed.
But how do these bacteria emit light An operon -a genetic operational unit that regulates a genetic product by controlling mRNA production-is responsible. In addition to containing more than one structural gene, operons have a regulatory gene, control sites, and an operator region that acts as an on/off switch. In bioluminescent bacteria, this is known as the lux operon, which controls production of luciferase , an enzyme that leads to the emission of light.
Why would arranging genes in an operon be advantageous to a bacterium
What stimulates a bacterium to produce light
The lux operon in Vibrio fischeri and Vibrio harveyi has been carefully studied. Vibrio fischeri is an organism that lives either on its own or symbiotically with the bobtail squid; Vibrio harveyi lives on shrimp and other marine species. The lux operon is composed of five different genes that control bioluminescence. Protein products of this operon combine to form luciferase and bioluminesce by emitting photons of light.
Because transcription and translation of the genes in the lux operon require energy, the cell only translates and transcribes these genes at certain times. Having the genes adjacent to one another and under the control of a signal mechanism is very efficient for the bacterium. The bacteria continually produce a signaling molecule called an autoinducer , which is not unlike a hormone. When the bacteria reach a certain concentration, the autoinducer reaches a concentration that is detectable by all of the bacteria in the immediate area. The higher concentration of the autoinducer signals the bacteria that they are at the right concentration to produce luciferase and bioluminesce. This phenomenon is known as quorum sensing. The bacteria do not produce light until they have reached a specific concentration. At low concentrations of bacteria, not enough autoinducer is produced, the signaling pathway is shut down, and the lux operon is turned off. When bacteria reach concentrations of up to 10 12 cells/ml, the autoinducer initiates a signaling pathway that turns on the lux operon.
How does understanding bioluminescent bacteria and the lux operon help us to understand other bacterial processes