In metazoans, cyclin-dependent kinase(CDK) dependent phosphorylation of the retinoblastoma Tudor suppressor protein (Rb) alleviates repression of E2F and thereby activates G1/S transcription. The cell size regulator Whi5 appears to be an analogous target of CDK activity during G1 phase [1].

1: CDK activity antagonizes Whi5, an inhibitor of G1/S transcription in yeast. Costanzo M, Nishikawa JL, Tang X, Millman JS, Schub O, Breitkreuz K, Dewar D, Rupes I, Andrews B, Tyers M; Cell. 2004;117:899-913. PMID:15210111

2: Cln3 activates G1-specific transcription via phosphorylation of the SBF bound repressor Whi5. de Bruin RA, McDonald WH, Kalashnikova TI, Yates J 3rd, Wittenberg C; Cell. 2004;117:887-898. PMID:15210110