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Sara Hanson

Colorado College
Molecular Biology
Origin and Evolution of Yeast Mating-Type Switching
Yeast mating-type switching, in which haploids are able to interconvert between cell types MATa and MATalpha to facilitate mating, is a complex molecular process comprised of several essential components and requiring multiple levels of regulation. The evolution of this process is still largely uncharacterized, with the well-studied systems in Saccharomyces cerevisiae and Schizosaccharomyces pombe arising seemingly independently without evidence for a less complex ancestral switching mechanism. Recently, we described mating-type switching in the methylotrophic yeasts Hansenula polymorpha and Pichia pastoris, where a simpler chromosomal inversion mechanism is used to convert mating-type. In these species, the mating-type (MAT) region contains both MATa and MATalpha genes and is flanked by inverted repeat sequences. One set of MAT genes is silenced by their proximity to a transcriptionally repressed centromere or telomere. Switching occurs by inversion of the MAT region between the repeat sequences resulting in the opposite pattern of MAT gene expression. Mating-type switching, mating, and sporulation in these species are induced by nitrogen limitation, although the molecular components underlying this response are unknown. Using targeted gene deletions, RNA-seq, and bulk segregant analysis, we identify candidate genes involved in the mating-type switching response in methylotrophic yeasts.
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