Why are essential genes essential? - The essentiality of Saccharomyces genes

Abstract

Essential genes are defined as required for the survival of an organism or a cell. They are of particular interests, not only for their essential biological functions, but also in practical applications, such as identifying effective drug targets to pathogenic bacteria and fungi. The budding yeast Saccharomyces cerevisiae has approximately 6,000 open reading frames, 15 to 20% of which are deemed as essential. Some of the essential genes, however, appear to perform non-essential functions, such as aging and cell death, while many of the non-essential genes play critical roles in cell survival. In this paper, we reviewed and analyzed the levels of essentiality of the Saccharomyces cerevisiae genes and have grouped the genes into four categories: (1) Conditional essential: essential only under certain circumstances or growth conditions; (2) Essential: required for survival under optimal growth conditions; (3) Redundant essential: synthetic lethal due to redundant pathways or gene duplication; and (4) Absolute essential: the minimal genes required for maintaining a cellular life under a stress-free environment. The essential and non-essential functions of the essential genes were further analyzed.

Keywords: Saccharomyces cerevisiae; cell death; cell survival; essential genes; growth conditions.

Figure 1. FIGURE 1: A symbolic illustration of different levels of essentiality of the Saccharomyces cerevisiae genes.

(1) Conditional essential: when all other roads are closed, this alternative road becomes essential. (2) Essential: the only available road to your destination. (3a) Redundant essential: two duplicated/similar roads to your destination; (3b) Redundant essential: two different roads (pathways), but to the same destination; (3c) Redundant essential (synthetic sick): Two similar/different roads, plus a less preferred road, when the main roads become unavailable. The last part of the road (marked as red) represents the absolute essential.

Figure 2. FIGURE 2: Involvement of essential genes (ESP1, MCD1 and BIR1) in apoptotic cell death. Modified according to .

Figure 3. FIGURE 3: TEM image of pds5-1 mutant showing the nucleus unable to divide when switched to non-permissive temperature.

It also shows chromatin condensation (arrowhead) and nucleus with protruding tube (arrow), both typical markers of apoptotic cell death. N = nucleus. M = mother cell, as suggested by the vacuole.