Elucidating the Regulation of Toxin Production In Naturally Occurring Algal-Bacterial Symbioses

Abstract

Roseobacticides are novel secondary metabolites with potent anti-algal activity. These molecules are produced by Phaeobacter gallaeciensis, a marine bacterium of the roseobacter clade, as a result of a complex symbiotic relationship with the microalga Emiliania huxleyi. In the mutualistic phase of this symbiosis, E. huxleyi provides food in the form of a carbon and sulfur source for the bacteria. In return, P. gallaeciensis produces the antibiotic tropodithietic acid and the growth hormone phenylacetic acid, to promote and protect the algal host. However, once the algae senesce and release lignin monomers, such as p-coumaric acid and sinapic acid, the interaction becomes parasitic. These putative senescence signals elicit the production of roseobacticides, potent algaecidal compounds that quickly kill the algae in less than 24 hours, by P. gallaeciensis. Elucidating the regulatory pathways that lead to the release of roseobacticides will aid in understanding the toxin production in this and other eukaryote-bacterial symbioses. In this study, I used genetic and transcriptomic analyses to examine the molecular networks underlying this mutualist-to-parasite switch. Specifically, I wanted to characterize the role of tdaA, a LysR-type transcriptional regulator required for the production of tropodithietic acid and roseobacticides. I found that genes in two distinct operons used for tropodithietic production are also employed in roseobacticide synthesis. Transcriptomic analyses revealed that the expression of these is increased 10-120 fold, when the bacterial cultures are supplemented with sinapic acid or p-coumaric acid. In addition, I showed for the first time that tdaA is the genetic “switch” that transforms P. gallaeciensis into an algal parasite. The results from my experiments provide a model for the regulation of roseobacticide production and the factors that control conversion of a bacterial mutualist to a host pathogen.