In the 2015-2016 academic year, we will explore the conservation of Met and Cys biosynthetic enzymes between the budding yeast, Saccharomyces cerevisiae, and the fission yeast,Schizosaccharomyces pombe. As their names imply, S. cerevisiae and S. pombe are sugar-loving fungi that were originally isolated from beer. S. pombe and S. cerevisiae are members of the phylum Ascomycota that can be propagated in both diploid and haploid forms. In response
to various stresses, haploid strains of opposite mating types are induced to mate and undergo meiosis. The four spores generated from meiosis are contained within a resistant structure known as the ascus, from which the phylum derives its name. The two species are thought to have diverged from a common ancestor about 1 billion years ago (Hedges, 2002). Since their divergence, the S. cerevisiae lineage has undergone a whole genome duplication, followed by rounds of gene elimination and diversification (Mortimer, 2000). Today, the size of the S. cerevisiae genome (Kellis et al.,2004), ~12.5 Mbp, is similar to that of S. pombe. Because it has undergone less genome diversification, S. pombe is considered to be much closer to ancestral members of the phylum.
Diversification of selected yeast species within the Phylum Ascomycota
Different mechanisms of cell replication in S. cerevisiae and S. pombe are apparent in electron micrographs. (S. cerevisiae image reproduced with permission of Christopher Buser. S. pombe image from Hochstenbachet al., Copyright National Academy of Sciences, U.S.A (1998), is reproduced with permission.)
Of the two yeasts, S. cerevisiae is far and away the more thoroughly studied. Scientists have worked with genetically pure strains of S. cerevisiae for over a century, and it is widely used as a model organism (Botstein and Fink, 2011). S. cerevisiae has many of the same biochemical pathways as higher eukaryotes, but its genome is significantly smaller than vertebrate genomes and powerful genetic techniques are available for manipulating gene expression. For these reasons, the S. cerevisiae genome was the first eukaryotic genome to be sequenced in its entirety. Completion of the yeast genome sequence (Goffeau et al., 1996) allowed researchers to prepare genome-wide collections of mutant strains (Winzeler et al., 1999) and plasmids (Gelperin et al., 2005) that are available to the yeast community. This semester, we will use S. cerevisiae strains with defined defects in Met and Cys biosynthesis as the hosts for homologous genes from S. pombe. If the S. pombe gene restores the ability of the S. cerevisiae mutant to synthesize Met, in a process known as complementation, we will know that gene function has been conserved over the evolutionary time that separates the two species.