Sage Brian T, Wu Michael D, Csink Amy K
Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
Genetics. 2008 Feb;178(2):749-59. doi: 10.1534/genetics.107.083105. Epub 2008 Feb 1.
The brown(Dominant) (bw(D)) allele of Drosophila contains a heterochromatic block that causes the locus to interact with centric heterochromatin. This association silences bw(+) in heterozygotes (trans-inactivation) and is dependent on nuclear organizational changes later in development, suggesting that trans-inactivation may not be possible until later in development. To study this, a P element containing an upstream activating sequence (UAS)-GFP reporter was inserted 5 kb from the bw(D) insertion site. Seven different GAL4 driver lines were used and GFP fluorescence was compared in the presence or the absence of bw(D). We measured silencing in different tissues and stages of development and found variable silencing of GFP expression driven by the same driver. When UAS-GFP was not expressed until differentiation in the eye imaginal disc it was more easily trans-inactivated than when it was expressed earlier in undifferentiated cells. In contrast to some studies by other workers on silencing in cis, we did not find consistent correlation of silencing with level of expression or evidence of relaxation of silencing with terminal differentiation. We suggest that such contrasting results may be attributed to a potentially different role played by nuclear organization in cis and trans position-effect variegation.
果蝇的棕色（显性）（bw（D））等位基因包含一个异染色质区，该区域导致该基因座与着丝粒异染色质相互作用。这种关联使杂合子中的bw（+）沉默（反式失活），并且依赖于发育后期的核组织变化，这表明反式失活可能直到发育后期才会发生。为了研究这一点，将一个含有上游激活序列（UAS）-绿色荧光蛋白（GFP）报告基因的P元件插入到距离bw（D）插入位点5 kb处。使用了七种不同的GAL4驱动系，并比较了在有或没有bw（D）的情况下GFP荧光。我们测量了不同组织和发育阶段的沉默情况，发现由同一驱动系驱动的GFP表达存在可变的沉默现象。当UAS-GFP直到眼成虫盘分化时才表达时，它比在未分化细胞中更早表达时更容易被反式失活。与其他研究人员关于顺式沉默的一些研究不同，我们没有发现沉默与表达水平之间的一致相关性，也没有发现随着终末分化沉默松弛的证据。我们认为，这种对比结果可能归因于核组织在顺式和反式位置效应斑驳中可能发挥的不同作用。
