Yeast for virus research

Abstract

Budding yeast (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) are two popular model organisms for virus research. They are natural hosts for viruses as they carry their own indigenous viruses. Both yeasts have been used for studies of plant, animal and human viruses. Many positive sense (+) RNA viruses and some DNA viruses replicate with various levels in yeasts, thus allowing study of those viral activities during viral life cycle. Yeasts are single cell eukaryotic organisms. Hence, many of the fundamental cellular functions such as cell cycle regulation or programed cell death are highly conserved from yeasts to higher eukaryotes. Therefore, they are particularly suited to study the impact of those viral activities on related cellular activities during virus-host interactions. Yeasts present many unique advantages in virus research over high eukaryotes. Yeast cells are easy to maintain in the laboratory with relative short doubling time. They are non-biohazardous, genetically amendable with small genomes that permit genome-wide analysis of virologic and cellular functions. In this review, similarities and differences of these two yeasts are described. Studies of virologic activities such as viral translation, viral replication and genome-wide study of virus-cell interactions in yeasts are highlighted. Impacts of viral proteins on basic cellular functions such as cell cycle regulation and programed cell death are discussed. Potential applications of using yeasts as hosts to carry out functional analysis of small viral genome and to develop high throughput drug screening platform for the discovery of antiviral drugs are presented.

Keywords: Saccharomyces cerevisiae; Schizosaccharomyces pombe; cell cycle regulation; genome-wide analysis; high throughput drug screening; programed cell death; viral replication; virus-host interaction.

Figure 1. FIGURE 1: Life cycles of budding yeast (Saccharomyces cerevisiae) (A) and fission yeast (Schizosaccharomyces pombe) (B).

Diagrams show yeasts have both asexual (vegetative) and sexual reproductive cycles, respectively. (A) Budding yeast is generally maintained in the laboratory through vegetative growth both as haplontic (haploid) and diplontic (diploid) cells during asexual life cycle by mitosis, which produce daughter cells by budding off of mother cells. Mitotic cell cycle has all of the typical eukaryotic cell cycle stages of G1, S, G2 and M phases, but it spends most of its cell cycling in G1 phase, which is similar to human cell cycle. Under stressful conditions, diploid cells undergo meiosis to form haploid spores by sporulation. Haploid cells of opposite mating types (a or α) can go on to mate (conjugate) and to reform diploid cells. (B) Fission yeast is normally present as haploid cells through mitosis. Cells are divided equally between daughter and mother cells. In contrast to budding yeast cell cycle, fission yeast spends most of its cell cycling in G2 phase. Its diploid form could be triggered by stress through mating of opposite mating type (h⁺ or h^-) during meiosis.

Figure 2. FIGURE 2: Schematic diagram of a shotgun approach to clone a small viral genome in fission yeast.

(Top) the pYZ1N vector contains a wild type nmt1 promoter . It carries a LEU2 gene for selection. pYZ2N contains the wild type nmt1 promoter and a URA4 gene selection marker. (Bottom) pYZ3N-GFP contains the same nmt1 promoter as pYZ1N and has an added green fluorescent protein GFP tag. The α-peptide of β-galactosidase is used for selecting DNA inserts with α-complementation. Unique cloning sites in these vectors are indicated. ars1, origin of replication from S. pombe; Leu2, Saccharomyces cerevisiae leucine biosynthesis gene; AmpR, bacterial ampicillin resistance gene. Adapted from . Note that nothing is new in molecular features of shuttle vectors described. Goal of this figure is to illustrate a robust and streamlined strategy of shotgun gene cloning of a small viral genome.