[3] Hartwell not only identified the mutant, cdc28, which arrests in very early stages of the cell cycle, but he also recognised that the presence of mating factors could result in similar phenotypes of inhibited bud formation and lack of DNA synthesis.
In the years following Hartwell's labor-intensive experiments, it has been shown that other environmental factors contribute to cellular fate in yeast and analogously in other organisms.
Though not yeast-specific, a critical study put forth by Zetterberg et al. in 1985 provided evidence for a commitment point in Swiss 3T3 cells, or mouse embryo fibroblasts, when grown in serum-rich or serum-starved conditions.
The export of Whi5 results in the partial activation of the transcription factors SBF and MBF, which ultimately promote cell cycle progression.
A modern-day study delineating the relationship between mating arrest and cell cycle progression was put forth by Doncic et al. in June 2011.
[6] Recognizing that the amount of nuclear Whi5 is an indicator of G1 cyclin activity, the authors set out to quantitatively understand the point at which cells commit to division.
Using a Whi5-GFP fusion protein, they tracked the amount of nuclear Whi5 following the addition of alpha-factor, and noted whether the cell arrested or continued division.
Thus, the differential response to the presence of pheromones is reflected in whether the cell is pre- or post-Start, states that can be characterized by how much Whi5 is present in the nucleus.
In 2008, Skotheim et al. proposed that this feedback loop allows for a strong signal to commit to cellular division by the SBF and MBF regulated genes.
[5] They hypothesized that without a coherent expression of the genes necessary for early events, like DNA replication and bud-site formation, random individual cellular signals creates noise that weakens the commitment response.
A Whi5 allele lacking six of twelve phosphorylation sites results in a slow exit from the nucleus, and consequently a less coherent induction of CLN2 and RAD27 expression.
Doncic et al. proposed that the incomplete division was due to expression of genes in both the mating pathway and in the G1 cyclin-driven cellular progression.