Overcoming telomeric silencing: a trans-activator competes to establish gene expression in a cell cycle-dependent way.
Genes located near telomeres in yeast are subject to position-effect variegation. To better understand the mechanism of this variegation, we investigated how a telomeric URA3 gene switches from a silent to an expressed state. We found that silencing of a telomeric URA3 gene was attributable to the elimination of its basal transcription. The reversal of that silencing was dependent on the presence of PPR1, the trans-activator protein of URA3. Maximum expression of URA3 required a higher concentration of PPR1 when URA3 was telomeric compared with when it was at a nontelomeric location. The ability of PPR1 to overcome silencing varied at different points in the cell cycle. In cells arrested in G2/metaphase, PPR1 was able to activate transcription of a telomeric URA3, but in cells arrested in G0, G1, or early S phase it was not. In comparison, a nontelomeric URA3 could be activated by PPR1 at all times. We conclude that once established, telomeric silent chromatin is a relatively stable structure, making a gene recalcitrant to activation. Following the disassembly of silent chromatin during DNA replication, competition of assembly ensues between components of telomeric chromatin, to establish a silent state, and the trans-activator, to establish gene expression. These results help explain the stochastic nature of phenotypic switching in variegated gene expression.
Aparicio
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Department of Molecular Genetics & Cell Biology, University of Chicago, Illinois 60637.
Gottschling
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D001483:Base Sequence; D002453:Cell Cycle / Q000235:genetics; D004268:DNA-Binding Proteins / Q000502:physiology*; D005656:Fungal Proteins / Q000235:genetics / Q000502:physiology; D015966:Gene Expression Regulation, Fungal / Q000235:genetics*; D008969:Molecular Sequence Data; D009964:Orotidine-5'-Phosphate Decarboxylase / Q000235:genetics; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000235:genetics*; D029701:Saccharomyces cerevisiae Proteins; D016615:Telomere / Q000235:genetics*; D015534:Trans-Activators / Q000502:physiology*; D014157:Transcription Factors / Q000502:physiology*
D004268:DNA-Binding Proteins; D005656:Fungal Proteins; C052130:GAL4 protein, S cerevisiae; C065682:PPR1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D009964:Orotidine-5'-Phosphate Decarboxylase