ArcA and FNR regulate bioluminescence in the light-organ symbiont Vibrio fischeri
Bose, Jeffrey L.
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Despite years of intense study of bioluminescence in Vibrio fischeri, much remains unanswered regarding its benefit to the cell and the environmental factors affecting its regulation. In this dissertation, I show that bioluminescence leads to a growth rate reduction under some culture conditions, yet it contributes to this bacterium’s ability to colonize one of its symbiotic hosts, the squid Euprymna scolopes. Based on the reaction catalyzed by the light-producing enzyme luciferase, two conflicting hypotheses have been suggested to explain the benefit that bioluminescence provides symbiotic V. fischeri. One proposal is that luminescence provides an advantage by consuming oxygen and thus serving as an antioxidant to protect the cell from toxic oxygen radicals. Conversely, a second proposal suggests that by consuming excess reductant luminescence acts as an electron sink. I examined the regulation of luminescence to help distinguish between these two hypotheses, assuming that luminescence would be maximally expressed when it is most beneficial. I examined luminescence of mutants lacking the redox-responsive and oxygen-sensitive regulators ArcA and FNR, respectively. This was performed in the well-studied and naturally visibly luminescent strain MJ1 as well as in the relatively dim strain ES114, which is a natural symbiont of E. scolopes. ArcA is most active under reducing conditions, and I found that it represses luminescence in both strains due to a conserved ArcA binding site in the PluxI promoter, which controls the expression of the proteins necessary for light production. Similar to ArcA, FNR is active under anaerobic conditions and represses luminescence in MJ1, although no such effect is apparent in ES114. These results demonstrate that luminescence is repressed under reduced conditions and therefore my data is consistent with the antioxidant model explaining the benefits of luminescence, but are inconsistent with the electron sink model. This work is the first to use defined mutants in V. fischeri to elucidate a mechanism by which specific environmental factors alter the regulation of luminescence.