OVERCOMING TELOMERIC SILENCING - A TRANSACTIVATOR COMPETES TO ESTABLISH GENE-EXPRESSION IN A CELL CYCLE-DEPENDENT WAY

Genes located near telomeres in yeast are subject to position-effect v ariegation. To better understand the mechanism of this variegation, we investigated how a telomeric URA3 gene switches from a silent to an e xpressed state. We found that silencing of a telomeric URA3 gene was a ttributable to the elimination of its basal transcription. The reversa l of that silencing was dependent on the presence of PPR1, the trans-a ctivator protein of URA3. Maximum expression of URA3 required a higher concentration of PPR1 when URA3 was telomeric compared with when it w as at a nontelomeric location. The ability of PPR1 to overcome silenci ng varied at different points in the cell cycle. In cells arrested in G(2)/metaphase, PPR1 was able to activate transcription of a telomeric URA3, but in cells arrested in G(0), G(1), or early S phase it was no t. In comparison, a nontelomeric UR43 could be activated by PPR1 at al l times. We conclude that once established, telomeric silent chromatin is a relatively stable structure, making a gene recalcitrant to activ ation. Following the disassembly of silent chromatin during DNA replic ation, competition of assembly ensues between components of telomeric chromatin, to establish a silent state, and the trans-activator, to es tablish gene expression. These results help explain the stochastic nat ure of phenotypic switching in variegated gene expression.