Cln1Cln2

Cln1 and Cln2 are paralogous G1 cyclins of the budding yeast Saccharomyces cerevisiae. They encode regulatory subunits that bind the cyclin-dependent kinase Cdc28 to form Cln1–Cdc28 and Cln2–Cdc28 complexes. These complexes promote progression through the G1 phase and entry into S phase, effectively driving the G1/S transition. Cln1 and Cln2 are largely redundant in function, and loss of both genes causes a severe delay or arrest in G1, whereas single mutants typically show only modest phenotypes.

During G1, Cln1/2–Cdc28 activity phosphorylates key substrates to relieve repression of SBF/MBF-driven transcription. In particular, phosphorylation of Whi5, a repressor of SBF, leads to its inactivation and nuclear export, enabling transcription of G1/S genes. Cln1/2 activity also facilitates activation of downstream S-phase processes by promoting degradation of CDK inhibitors such as Sic1, thereby permitting accumulation of S-phase cyclins. In many conditions, Cln3 acts upstream to initiate the G1 program, with Cln1/2 maintaining and amplifying the signal for START.

Regulation: Expression of CLN1 and CLN2 is cell cycle–regulated, arising in early to mid-G1 under control of SBF/MBF-responsive promoters and subject to feedback by the same transcriptional network. Their activities are modulated by inhibitors and substrate availability, including phosphorylation events that target proteins like Sic1 for degradation, enabling Cln1/2–Cdc28 activity. The two genes are functionally redundant, allowing partial compensation if one is lost.

Significance: Cln1 and Cln2 are classic model regulators of eukaryotic cell cycle control, central to understanding START, CDK regulation, and the transcriptional control of G1/S genes in yeast.