Development

Stochastic Mosaics

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Science  13 Apr 2001:
Vol. 292, Issue 5515, pp. 173
DOI: 10.1126/science.292.5515.173a

Position effect variegation (PEV) in the fruit fly Drosophila is the silencing of gene expression caused by the proximity of a gene to heterochromatin (a condensed and transcriptionally inactive region of the chromosome). One intriguing feature of PEV is mosaicism, where an affected gene may be silenced in one cell but active in a neighboring, and apparently identical, cell. Ahmad and Henikoff have used a reporter gene that is subject to PEV and has binding sites for the GAL4 activator in its promoter region to tease apart the basis of PEV and mosaicism. By modulating GAL4 activator expression during development, they show that promoter activation can counteract the silenced state of the reporter, presumably by binding to and trapping transiently exposed GAL4 binding sites in the heterochromatin. Mosaicism then is a consequence of the spontaneous and stochastic derepression of the promoter in the presence of the activator. — GR

Cell104, 839 (2001).

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