Proline-rich peptide from the coral pathogen Vibrio shiloi that inhibits photosynthesis of zooxanthellae

The coral-bleaching bacterium Vibrio shiloi biosynthesizes and secretes an extracellular peptide, referred to as toxin P, which inhibits photosynthesi s of coral symbiotic algae (zooxanthellae). Toxin P was produced during the stationary phase when the bacterium was grown on peptone or Casamino Acids media at 29 degreesC. Glycerol inhibited the production of toxin P. Toxin P was purified to homogeneity, yielding the following 12-residue peptide: P YPVYAPPPVVP (molecular weight, 1,295.54). The structure of toxin P was conf irmed by chemical synthesis. Tn the presence of 12.5 mM NH4Cl, pure natural or synthetic toxin P (10 muM) caused a 64% decrease in the photosynthetic quantum yield of zooxanthellae within 5 min. The inhibition nas proportiona l to the toxin P concentration. Toxin P bound avidly to zooxanthellae, such that subsequent addition of NK,CI resulted in rapid inhibition of photosyn thesis. When zooxanthellae were incubated in the presence of NH,CI and toxi n P, there was a rapid decrease in the pH (pH 7.8 to 7.2) of the bulk liqui d, suggesting that toxin P facilitates transport of NH, into the cell, It i s known that uptake of NH, into cells can destroy the pH gradient and block photosynthesis. This mode of action of toxin P can help explain the mechan ism of coral bleaching by V.shiloi.