POSITION EFFECT VARIEGATION IN YEAST

Classically, position effect variegation has been studied in Drosophil a and results when a euchromatic gene is placed adjacent to either cen tromeric heterochromatin or to a telomeric domain. In such a circumsta nce expression of the locus variegates, being active in some cells and silent in others. Over the last few years a comparable phenomenon in yeast has been discovered. This system promises to tell us much about this curious behavior. Indeed, experiments reported recently((1)) indi cate that the variegation of a yeast telomeric gene is cell-cycle regu lated. The results suggest the following model. During DNA replication there is a disassembly of chromatin that allows a competition between silencing factors and transactivators to take place. Thus, reassembly of the domain may result in either the repression or the expression o f the affected gene and, hence, produce a variegating phenotype.