Biosynthesis and Genetic Tuning of Guanine Crystals via Metabolic Engineering of Yeast.

Purine crystals, such as guanine, generate diverse optical phenomena in animals and function as important nitrogen-storage reservoirs in microalgae. Here, we demonstrate the formation of guanine crystals in Brewer's yeast. Whilst guanine crystals do not naturally contribute to the physiology of this yeast species, we show how crystallization may be artificially induced and then use genetic manipulation to control crystal properties. Specifically, we use genetic knockouts of purine salvage enzymes to modify crystal morphology, size and crystalline order. Deletion of Hpt1 and Gud1, which convert guanine to guanosine monophosphate and xanthine, respectively, transforms spherical guanine nanoparticles in wild-type cells into crystalline dumbbells and nanoflowers. While we are lacking a definitive mechanism to rationalize this transformation, we hypothesize that changes in crystal morphology are likely driven by genetically induced shifts in the local supersaturation and metabolite composition within the yeast vacuole. Leveraging yeast's genetic programmability and ubiquitous industrial usage, our findings provide a powerful biotechnological strategy for the biosynthesis of optically functional guanine crystals, as environmentally friendly alternatives to toxic pigments in a range of industries.