👤 Lorenzo Querin

The FAR1 gene encodes an 830 residue bifunctional protein, whose major function is inhibition of cyclin-dependent kinase complexes involved in the G1/S transition. FAR1 transcription is maximal between mitosis and early G1 phase. Enhanced FAR1 transcription is necessary but not sufficient for the pheromone-induced G1 arrest, since FAR1 overexpression itself does not trigger cell cycle arrest. Besides its well established role in the response to pheromone, recent evidences suggest that Far1 may also regulate the mitotic cell cycle progression: in particular, it has been proposed that Far1, together with the G1 cyclin Cln3, may be part of a cell sizer mechanism that controls the entry into S phase. Far1 is an unstable protein throughout the cell cycle except during G1 phase. Far1 levels peak in newborn cells as a consequence of a burst of synthetic activity at the end of the previous cycle, and the amounts per cell remain roughly constant during the G1 phase. Phosphorylation (at serine 87) by Cdk1-Cln complexes primes Far1 for ubiquitin-mediated proteolysis. By coupling a genome-wide transcriptional analysis of FAR1-overexpressing and far1Δ cells grown in ethanol- or glucose-supplemented minimal media with a range of phenotypic analysis, we show that FAR1 overexpression not only coordinately increases RNA and protein accumulation, but induces strong transcriptional remodeling, metabolism being the most affected cellular property, suggesting that the Far1/Cln3 sizer regulates cell growth either directly or indirectly by affecting metabolism and pathways known to modulate ribosome biogenesis. A crucial role in mediating the effect of Far1 overexpression is played by the Sfp1 protein, a key transcriptional regulator of ribosome biogenesis, whose presence is mandatory to allow a coordinated increase in both RNA and protein levels in ethanol-grown cells. Show less

Saccharomyces cerevisiae cells grown in glucose have larger average size than cells grown in ethanol. Besides, yeast must reach a carbon source-modulated critical cell size in order to enter S phase at Start. This control is of outmost physiological relevance, since it allows us to coordinate cell growth with cell cycle progression and it is responsible for cell size homeostasis. The cell sizer mechanism requires the overcoming of two sequential thresholds, involving Cln3 and Far1, and Clb5,6 and Sic1, respectively. When both thresholds are non-functional, carbon source modulation of cell size at Start is completely abolished. Since inactivation of extracellular glucose sensing through deletion of either the GPR1 or the GPA2 gene causes a marked, but partial, reduction in the ability to modulate cell size and protein content at Start, it is proposed that both extracellular and intracellular glucose signalling is required for properly setting the cell sizer in glucose media. Show less

Saccharomyces cerevisiae must reach a carbon source-modulated critical cell size, protein content per cell at the onset of DNA replication (Ps), in order to enter S phase. Cells grown in glucose are larger than cells grown in ethanol. Here, we show that an increased level of the cyclin-dependent inhibitor Far1 increases cell size, whereas far1 Delta cells start bud emergence and DNA replication at a smaller size than wild type. Cln3 Delta, far1 Delta, and strains overexpressing Far1 do not delay budding during an ethanol glucose shift-up as wild type does. Together, these findings indicate that Cln3 has to overcome Far1 to trigger Cln-Cdc28 activation, which then turns on SBF- and MBF-dependent transcription. We show that a second threshold is required together with the Cln3/Far1 threshold for carbon source modulation of Ps. A new molecular network accounting for the setting of Ps is proposed. Show less