The transcription factor Swi4 is target for PKA regulation of cell size at the G1 to S transition in Saccharomyces cerevisiae

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 (cyr1 Delta, pde2 Delta, msn2 Delta, msn4 Delta). 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(1), followed by an accumulation of cells in G(2)/M phase with a minimal change in the growth rate. Deletion of CLN1 gene, but not of CLN2, abolished the transient G(1) phase arrest. Consistently we found that PKA activation caused a transcriptional repression of CLN1 gene. Transcription of CLN1 is controlled by SBF and MBF dual-regulated promoter. We found that also the deletion of SWI4 gene abolished the transient G(1) arrest suggesting that Swi4 is a target responsible for PKA modulation of G(1)/S phase transition. We generated a SWI4 allele mutated in the consensus site for PKA (Swi4(S159A)) and we found that expression of Swi4(S159A) protein in the GG104-Swi4 Delta strain did not restore the transient G(1) arrest induced by PKA activation, suggesting that Swi4 phosphorylation by PKA regulates CLN1 gene expression and G(1)/S phase transition.