Regulation of Cell Size by Glucose Is Exerted via Repression of the CLN1 Promoter

ABSTRACT

Yeast cells are keenly sensitive to the availability and quality of nutrients. Addition of glucose to cells growing on a poorer carbon source elicits a cell cycle delay during G₁ phase and a concomitant increase in the cell size. The signal is transduced through the RAS-cyclic AMP pathway. Using synchronized populations of G₁ cells, we show that the increase in cell size required for budding depends upon CLN1 but not other G₁ cyclins. This delay in cell cycle initiation is associated specifically with transcriptional repression of CLN1. CLN2 is not repressed. Repression of CLN1 is not limited to the first cycle following glucose addition but occurs in each cell cycle during growth on glucose. A 106-bp fragment of the CLN1 promoter containing the three MluI cell cycle box (MCB) core elements responsible for the majority ofCLN1-associated upstream activation sequence activity is sufficient to confer glucose-induced repression on a heterologous reporter. A mutant CLN2 promoter that is rendered dependent upon its three MCB core elements due to inactivation of its Swi4-dependent cell cycle box (SCB) elements is also repressed by glucose. The response to glucose is partially suppressed by inactivation of SWI4, but not MBP1, which is consistent with the dependence of MCB core elements upon the SCB-binding transcription factor (SBF). We suggest that differential regulation of CLN1 and CLN2 by glucose results from differences in the capacity of SBF to activate transcription driven by SCB and MCB core elements. Finally, we show that transcriptional repression is sufficient to explain the cell cycle delay that occurs in response to glucose.

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Regulation of Cell Size by Glucose Is Exerted via Repression of the CLN1 Promoter

ACKNOWLEDGMENTS

The first two authors contributed equally to this work.

We thank Steve Haase and Peter Kaiser for careful reading of the manuscript and helpful suggestions. We also thank Linda Breeden, Warren Heideman, and M. David Baroni for communication of results prior to publication and for stimulating discussion. We are grateful to Linda Breeden, Steve Elledge, and Mike Tyers for plasmids and Stefan Lanker for development and maintenance of software for analysis of Coulter Channelyzer data.

We acknowledge support for D.C.-S. by a fellowship from the U.S. Public Health Service (NIH F32 GM16857). D. Stuart is the recipient of a Special Fellowship from the Leukemia Society of America. This work was supported by U.S. Public Health Service grants GM43487 and GM46006 from the National Institutes of Health.