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Sigma Deletion Mutant Collection

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Owen Ryan, Doris Cheung

Systematic phenotypic and genetic analysis with the yeast S. cerevisiae deletion mutant collection has proven incredibly powerful as it enables researchers to examine mutations in each gene for specific phenotypes comprehensively. To further expand the use of the deletion mutant collection, we developed a system for moving it from the standard laboratory strain genetic background, S288C, to a wild-type background, Sigma 1278b (Fig 9), which carries thousand of polymorphisms when compared to S288c [Winzeler et al., Genetics 163: 79-89 (2003)] and is capable of filamentous growth [Liu et al., Science 262:1741-1744 (1993)], a complex developmental process of relevance to fungal pathogenesis.

Many fungal species are capable of existing in two distinct developmental forms, a single celled yeast form and a multicellular filamentous form. The transition between the two forms is termed the dimorphic switch and this switch is initiated by environmental stimuli. The dimorphic transition has clinical relevance, as the most common human fungal pathogen Candida albicans is virulent in part through its capacity to reversibly switch between the yeast and filamentous forms. With rates of systemic Candida infections (candidiasis) on the rise, with mortality rates of over 40%, it has become increasingly more important to study the mechanisms that lead to dimorphic switching. Because S. cerevisiae is also dimorphic, it provides a powerful model organism to study this developmental switch.

Figure 9

Sigma 1278b Deletion Strain Construction

So far, we have moved virtually all bar-coded deletion mutant alleles from the S288C genetic background into the filamentous-competent S. cerevisiae Sigma 1278b background, generating approximately 16,000 deletion mutants. The Sigma 1278b deletion collection consists of a set of heterozygous diploid, MATa and MATa haploid, and homozygous diploid mutants. Interestingly, a number of genes are uniquely lethal to either the S288C or Sigma 1278b genetic backgrounds, suggesting that some of the polymorphic differences between the strains may be due to synthetic lethal or suppressive genetic interactions.

There are several developmental states associated with virulence in C.albicans. The ability to form invasive filaments, adhere to inert surfaces, and form fungal biofilms are all associated with the onset of systemic candidiasis. Sigma 1278b is capable of forming each of these developmental states and all Sigma 1278b deletion mutant strains are being studied systematically for defects in these processes. The result will be a comprehensive genetic analysis of fungal dimorhism. Data generated by the analyses in S.cerevisiae will give insights into homologous genes in C.albicans that are required for pathogenicity.