Wild yeast aren’t picky about their mates. For Saccharomyces cerevisiae, setting the mood is as simple as providing an abundant supply of nutrients, which prompts each yeast cell to search for another of the opposite mating type. If a lonesome yeast cell can’t find a suitable partner, it’s no problem—it can alternate between mating types, if needed, each cell division.

In contrast, lab yeast can’t pull off this trick; they carry deletions or other disabling mutations in parts of their genome needed for mating-type switching. Their inability to switch helps prevent the yeast from inconveniently changing types in the middle of an experiment, but it also creates a lot of extra work for geneticists when they’re trying to construct new strains while maintaining strict isogenicity. Now, researchers have developed a CRISPR/Cas9-based method to rapidly and efficiently switch mating types in a single cotransformation.

Mating-type switching is essential for constructing strains that are genetically identical except for the mating-type locus. These isogenic haploid strains are useful because they can mate with each other to form a diploid cell with two identical copies of the genome. And since diploid cells differ in many ways from their haploid equivalents, it’s often important to study genetic phenomena in both.

The ability to induce yeast to switch mating types is a particularly vital tool in the assembly of Sc2.0: a completely synthetic yeast genome. Creating and subsequently studying this genome could answer deep questions about biology—like how far we can go in modifying a eukaryote’s genome while still keeping the organism alive.

Completing this massive project, which is led by some of the authors of this study along with their collaborators around the world, will require many mating-type switches to consolidate the final synthetic chromosomes in a single strain. Switching the mating types of lab strains used to require a process with many steps, including manual dissection of spores using an exceptionally fine needle under a microscope. Having to rely on this classic but laborious process would slow the progress of the Sc 2.0 project, so the faster and easier CRISPR-Cas9-based technique offers a major advantage.

Although construction of the synthetic yeast genome isn’t yet in its final stages, many other yeast geneticists stand to benefit from the group’s speedy new technique, which would also be useful for building isogenic pairs of strains and even building isogenic tetraploids. Plus, freeing up geneticists from tedious tasks gives them more time to dream up schemes for extraordinary advances—like a whole genome built from scratch.

CITATION:

Xie, Z.; Mitchell, L.; Liu, H.; Li, B.; Liu, D.; Agmon, N.; Wu, Y.; Li, X.; Zhou, X.; Li, B.; Xiao, W.; Ding, M.; Wang, Y.; Yuan, Y.; Boeke, J. Rapid and Efficient CRISPR/Cas9-Based Mating-Type Switching of Saccharomyces cerevisiae.
G3, 8(1), 173-183.
DOI: 10.1534/g3.117.300347
http://www.g3journal.org/content/8/1/173

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Nicole Haloupek is a freelance science writer and a recent graduate of UC Berkeley's molecular and cell biology PhD program.

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