Using light for energy: examining the evolution of phototrophic metabolism via synthetic construction

Evolutionary innovations helped transform life on Earth. The origin of phototrophy was pivotal to increasing biomass, by utilizing light-driven energy transport to drive biological processes. Retinalophototrophy constitutes one of two phototrophic pathways on Earth, consisting of a simple system of microbial rhodopsins which are exemplars of horizontal gene transfer. Here, we seek to determine if Saccharomyces cerevisiae, a heterotrophic fungus, can function as a facultative artificial phototroph after acquiring only one rhodopsin gene. We investigate the fitness effects of turning yeast phototrophic by using synthetic biology to insert U. maydis rhodopsin into the vacuole of budding yeast. We observed a selective advantage in unicellular yeast with faster growth of yeast bearing rhodopsin in the presence of green light. This simple model illustrates the capacity of horizontal gene transfer to have a large phenotypic effect, with one gene enabling the transfer of phototrophic energy production into a modern eukaryote. While subject to the constraints of any given organism, this illustrates how single horizontal gene transfer events may be used to alter ecological niches and circumvent fitness constraints.