Hypersensitive SSY1 mutations negatively influence transition to quiescence in yeast Saccharomyces cerevisiae

Most cells spend the majority of their life in the non-proliferating, quiescent state. Transition to this state is crucial for microorganisms to survive long starvation periods and restart divisions afterwards. Experimental evolution allowed us to identify several mutation in genes that are presumably important for such transition in yeast cells. Most of these candidate genes belong to the SPS amino acid sensing pathway or to the SIR complex. We assembled these mutations on the ancestral strain background. Analysis of the quiescent/non-quiescent cell ratio of the starved yeast populations confirmed the crucial role of SSY1, the primary receptor component of the SPS sensor, in transition to the Q state. The evolved SSY1 mutations increased yeast sensitivity to amino acid presence in the environment. This resulted in decreased quiescent cell fraction and a 5.14% increase of the total amino acid content in the starved populations. We discuss external amino acid sensing via the SPS pathway as one of the mechanisms influencing transition to quiescence.