Exploring the growth to quiescence transition
When yeast naturally exhaust the glucose from their medium, they undergo one more division, which is highly asymmetric, and there is a slowing of physical growth. This results in a dramatic change in modal cell volume. These cells arrest in G1, undergo a global 30-fold drop in transcription and fortify their cell walls. This unique program of G1 arrest, asymmetric cell division, chromatin re-programming and cell wall fortification leads to the production of distinct cell types in stationary phase cultures that can be distinguished by flow cytometry. Using fluorescence-activated cell sorting, we showed that only one of these cell types has the properties of quiescent (Q) cells. We have explored the timing of the log to Q transition using flow cytometry and we have used a high throughput flow cytometry screen of a deletion library to identify mutants that fail to G1 arrest and enter quiescence.
Mechanisms driving chromatin to a quiescent state
Just as in log phase cells, the Cln3 cyclin must be down-regulated to achieve G1 arrest. The replication stress checkpoint is active during this interval, and it becomes essential for G1 arrest and viability if Cln3 is over-produced. The transcription repressor Xbp1 is induced after the glucose is exhausted from the medium (referred to as the diauxic shift or DS), and it represses CLN3 and hundreds of other transcripts after the DS. In the absence of Xbp1, cells undergo additional cell divisions. The resulting dense Q cells are very small and both their longevity and their recovery are compromised. Xbp1 recruits Rpd3 to its promoter binding sites, where it deacetylates histones and is specifically responsible for the global transcriptional shut down that occurs in quiescent cells.