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Transcriptional Silencing of the Yeast Mating-type Genes

Scott G. Holmes, Miriam Braunstein, James R. Broach


Differentiated cells rely on stable patterns of gene expression to maintain their committed state. Stability of gene expression in committed cell lineages depends on the interaction of a complex set of activating and repressing complexes with the DNA template. In each new cell division, these interactions must be precisely re-formed. One mechanism to ensure the stable inheritance of gene expression is to have the expression state of a gene play a role in its propagation. In this strategy, an activating or repressing complex acts as a template for its own re-formation through multiple cell divisions. Depending on the nature and regulation of the initial establishment of the expression state, this could lead to identical loci within the same cell having distinct, stable expression states.

Self-templating regulation has been proposed to explain the epigenetic pattern of inheritance of repressed transcription in organisms that exhibit heterochromatin-mediated position effects. Position effect refers to a phenomenon in which the expression state of a gene is influenced by its position on a chromosome. First described in Drosophila from adventitious juxtaposition of actively transcribed genes in euchromatic regions with a repressive heterochromatic domain (Henikoff 1990; Wilson et al. 1990), position effect resulting from heterochromatin also functions in many organisms to control regional gene expression in a regulated manner. A striking example is provided by dosage compensation in mammals. Female mammals have two copies of the X chromosome, whereas males have one. To prevent deleterious gene dosage effects, one X chromosome is inactivated in females. This is...

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