The transcription factor Swi4 is target for PKA regulation of cell size at the G<sub>1</sub> to S transition in <i>Saccharomyces cerevisiae</i>
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To investigate the specific target of PKA in the regulation of cell cycle progression and cell size we developed a new approach using the yeast strain GG104 bearing a deletion in adenylate cyclase gene and permeable to cAMP ( <i>cyr1</i>Δ, <i>pde2</i>Δ, <i>msn2</i>Δ, <i>msn4</i>Δ). In this strain the PKA activity is absent and can be activated by addition of cAMP in the medium, without any other change of the growth conditions. In the present work we show that the activation of PKA by exogenous cAMP in the GG104 strain exponentially growing in glucose medium caused a marked increase of cell size and perturbation of cell cycle with a transient arrest of cells in G<sub>1</sub>, followed by an accumulation of cells in G<sub>2</sub>/M phase with a minimal change in the growth rate. Deletion of <i>CLN1</i> gene, but not of <i>CLN2</i>, abolished the transient G<sub>1</sub> phase arrest. Consistently we found that PKA activation caused a transcriptional repression of <i>CLN1</i> gene. Transcription of <i>CLN1</i> is controlled by SBF and MBF dual-regulated promoter. We found that also the deletion of <i>SWI4</i> gene abolished the transient G<sub>1</sub> arrest suggesting that Swi4 is a target responsible for PKA modulation of G<sub>1</sub>/S phase transition. We generated a <i>SWI4</i> allele mutated in the consensus site for PKA (Swi4<sup>S159A</sup>) and we found that expression of Swi4<sup>S159A</sup> protein in the GG104-<i>Swi4</i>Δ strain did not restore the transient G<sub>1</sub> arrest induced by PKA activation, suggesting that Swi4 phosphorylation by PKA regulates <i>CLN1</i> gene expression and G<sub>1</sub>/S phase transition.
