Comparison of the Saccharomyces cerevisiae G1 cyclins: Cln3 may be an upstream activator of Cln1, Cln2 and other cyclins

EMBO J. 1993 May;12(5):1955-68.


In the budding yeast Saccharomyces cerevisiae, the G1 cyclins Cln1, Cln2 and Cln3 regulate entry into the cell cycle (Start) by activating the Cdc28 protein kinase. We find that Cln3 is a much rarer protein than Cln1 or Cln2 and has a much weaker associated histone H1 kinase activity. Unlike Cln1 and Cln2, Cln3 is not significantly cell cycle regulated, nor is it down-regulated by mating pheromone-induced G1 arrest. An artificial burst of CLN3 expression early in G1 phase accelerates Start and rapidly induces at least five other cyclin genes (CLN1, CLN2, HCS26, ORFD and CLB5) and the cell cycle-specific transcription factor SWI4. In similar experiments, CLN1 is less efficient than CLN3 at activating Start. Strikingly, expression of HCS26, ORFD and CLB5 is dependent on CLN3 in a cln1 cln2 strain, possibly explaining why CLN3 is essential in the absence of CLN1 and CLN2. To explain the potent ability of Cln3 to activate Start, despite its apparently weak biochemical activity, we propose that Cln3 may be an upstream activator of the G1 cyclins which directly catalyze Start. Given the large number of known cyclins, such cyclin cascades may be a common theme in cell cycle control.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Base Sequence
  • Cyclins / genetics
  • Cyclins / metabolism*
  • Fungal Proteins / genetics
  • Fungal Proteins / metabolism*
  • G1 Phase
  • Gene Expression Regulation, Fungal
  • Mating Factor
  • Molecular Sequence Data
  • Mutation
  • Oligodeoxyribonucleotides
  • Peptides / metabolism
  • Protamine Kinase / metabolism
  • RNA, Messenger / metabolism
  • Saccharomyces cerevisiae / cytology
  • Saccharomyces cerevisiae / genetics
  • Saccharomyces cerevisiae / metabolism*
  • Saccharomyces cerevisiae Proteins*
  • Transcription, Genetic
  • Transcriptional Activation


  • CLN3 protein, S cerevisiae
  • Cyclins
  • Fungal Proteins
  • Oligodeoxyribonucleotides
  • Peptides
  • RNA, Messenger
  • Saccharomyces cerevisiae Proteins
  • Mating Factor
  • Protamine Kinase