Whi3 binds the mRNA of the G(1) cyclin CLN3 to modulate cell fate in budding yeast

Eukaryotic cells commit in G(1) to a new mitotic cycle or to diverse differ entiation processes. Here we show that Whi3 is a negative regulator of Cln3 , a G(1) cyclin that promotes transcription of many genes to trigger the G( 1)/S transition in budding yeast. Whi3 contains an RNA-recognition motif th at specifically binds the CLN3 mRNA, with no obvious effects on Cln3 levels , and localizes the CLN3 mRNA into discrete cytoplasmic foci. This is the f irst indication that G(1) events may be regulated by locally restricting th e synthesis of a cyclin. Moreover, Whi3 is also required for restraining Cl n3 function in meiosis, filamentation, and mating, thus playing a key role in cell fate determination in budding yeast.