PubMed · 1579160
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J J Li, B M Alberts. 1992-05-14. DNA replication. Eukaryotic initiation rites.. https://doi.org/10.1038/357114a0
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Growth of S. cerevisiae cells by budding gives rise to asymmetric progeny cells: a larger "mother" cell and a smaller "daughter" cell. The mother cell transits a brief G1 phase before forming a new bud and beginning DNA replication. The daughter cell stays in G1 for a longer period, growing in size before initiating a new cell cycle. We show that the timing of cell cycle initiation in mother and daughter cells is governed by different G1 cyclins. In daughter cells, transcription of CLN1 and CLN2 is induced in a size-dependent manner, and these cyclins are necessary for the normal timing of cell cycle initiation. CLN3 is not required in daughter cells, but is crucial for mother cells, in which the G1 phase is much longer in the absence of this cyclin.
Two families of cyclin-like proteins have been found in S. cerevisiae. The clb proteins are the mitotic cyclins. The cln proteins provide an essential function, are required for the G1/S transition, and appear to be rate-limiting for START, but have no obvious role elsewhere in the cycle. The cln proteins are unstable; they form complexes with cdc28; the complexes have protein kinase activity; and at least one of the clns oscillates in abundance through the cell cycle. The action of the cln cyclins at START suggests that they may be 'G1 cyclins'.