Abstract
Light is a double-edged sword: it is essential for life on the planet but also causes cellular damage and death. Consequently, organisms have evolved systems not only for harvesting and converting light energy into chemical energy but also for countering its toxic effects. Despite the omnipresence and importance of such light-dependent effects, there are very few unbiased genetic screens, if any, investigating the mechanistic consequences that visible light has on cells. Baker's yeast, Saccharomyces cerevisiae, is one of the best annotated organisms thanks to several easily available mutant collections and its amenability to high-throughput genetic screening. However, until recently this yeast was thought to lack receptors for visible light, therefore its response to visible light was poorly understood. Nevertheless, a couple of years ago it was discovered that yeast senses light via a novel and unconventional pathway involving a peroxisomal oxidase, hydrogen peroxide, and a particular type of antioxidant protein, called peroxiredoxin. Here, we describe in detail a protocol for scoring yeast genes involved in the resistance to visible light (400-700 nm) on a genome-wide scale. Because cells in dense cultures shield each other from light exposure, resulting in apparent light resistance, our method involves adaptations to reduce inoculum size under conditions amenable to high-throughput screens, to properly be able to identify light-sensitive mutants. We also describe how to measure growth in the presence of light, including two follow-up validation tests. In this way, this method makes it possible to score light-sensitivity on a genome-wide scale with high confidence. Graphic abstract: Overview of strategy for high-throughput determination of yeast growth upon visible light stress.