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6 Telomere Position Effects and Transcriptional Silencing in the Yeast Saccharomyces cerevisiae

David Shore


The term “position effect” refers to differences in either gene activity, replication, or recombination that can be ascribed to the location of a particular DNA sequence within the chromosome. In the course of many decades of genetic research using the fruit fly Drosophila melanogaster, a large number of such events (both stable and unstable) have been identified and characterized (for a historical review, see Henikoff 1990). Stable position effects in Drosophila (and other eukaryotes) are now seen quite frequently due to the development of germ-line transformation and can be used to identify developmentally regulated genes (Wilson et al. 1990). These stable alterations in gene activity brought about by a novel chromosomal environment appear to be due to interactions between endogenous regulatory elements (e.g., tissue-specific enhancers) and the promoter of the affected gene. Unstable, or variegated, position effects, first described by Muller (1930), are often associated with the translocation of a euchromatic gene to a location near or within a region of heterochromatin (Schultz 1936). In the case studied by Muller, translocation of the white gene of Drosophila to a region near centromeric heterochromatin resulted in flies with a mixture of red and white patches of cells within the compound eye, due to either expression or repression of the gene within individual cells. Although position effect variegation (PEV) in Drosophila has been known and studied for well over half a century, its molecular mechanism remains unexplained. The phenomenon of PEV appears to be very widespread, since it has been observed in...

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