Regulators of Synthesis and Activity of the G₁ Cyclins of Budding Yeast

Excerpt

G₁ cyclins are known to play a crucial role in regulation of the cell cycle in the budding yeast, Saccharomyces cerevisiae. They are necessary for cell cycle progression (Richardson et al. 1989), and they appear to be the ultimate targets for cell cycle arrest by the negative growth factors, a-factor and α-factor (Fig. 1) (Chang and Herskowitz 1990; Wittenberg et al. 1990). Three G₁ cyclins—CLN1, CLN2, and CLN3—have been identified (Nash et al. 1988; Hadwiger et al. 1989). Western analysis demonstrates that the CLN2 protein is present in the cell cycle only in G₁ and that the CLN1 and CLN2 transcripts are present only in G₁ (Wittenberg et al. 1990). The three G₁ cyclins are functionally redundant in the sense that any one suffices for cell growth (Richardson et al. 1989): Mutants defective in all three genes arrest in G₁, at the same position in the cell cycle as mutants...