Growth-phase-dependent control of rRNA synthesis in Saccharomyces cerevisiae.

Saccharomyces cerevisiae is one of the most well-studied model organisms used in the scientific community. Its ease of manipulation, accessible growth conditions, short life cycle, and conserved eukaryotic metabolic pathways make it a useful model organism. Consequently, yeast has been used to investigate a myriad of phenomena, from microbial to human studies. Most of the research performed using this model organism utilizes yeast cell populations when they are growing exponentially, a growth phase aptly termed exponential or log phase. However, log phase encompasses several yeast generations and ranges several hours of yeast growth, meaning that there is a potential for variability during this "homogenous" growth phase. Cells in log phase require robust ribosome biogenesis to support their rapid growth and cell division. Interestingly, during log phase, ribosomal RNA (rRNA) synthesis (which is the first and rate limiting step in ribosome biosynthesis) has been shown to decrease prior to growth rate decline in stationary phase. In this study, we utilized several genomic and biochemical methods to elucidate the relationship between subphases of log phase and rRNA synthesis. Our results indicate that as yeast cells progress through subphases of log growth, both polymerase I transcription and rRNA processing are repressed. Overall, this study establishes a growth-phase-dependent control of rRNA synthesis that unexpectedly begins prior to the switch to stationary phase (i.e., pre-diauxic shift) as a putative mechanism of anticipating nutrient starvation.IMPORTANCESaccharomyces cerevisiae is a ubiquitously used model organism in a wide range of scientific research fields. The conventional practice when performing yeast studies is to investigate its properties during logarithmic growth phase. This growth phase is defined as the period during which the cell population doubles at regular intervals, and nutrients are not limiting. However, this growth phase lasts hours and encompasses several yeast cell generations which consequently introduce heterogeneity to log growth phase depending on their time of harvest. This study reveals significant changes in the transcriptomic landscape even in early stages of exponential growth. The overall significance of this work is the revelation that even the seemingly homogenous log growth phase is far more diverse than was previously believed.