Chan C S, Rastelli L, Pirrotta V
Department of Zoology, University of Geneva, Switzerland.
EMBO J. 1994 Jun 1;13(11):2553-64. doi: 10.1002/j.1460-2075.1994.tb06545.x.
Segmentation genes provide the signals for the activation and regulation of homeotic genes in Drosophila but cannot maintain the resulting pattern of expression because their activity ceases halfway through embryogenesis. Maintenance of the pattern is due to the Polycomb group of genes (Pc-G) and the trithorax group of genes (trx-G), responsible for the persistence of the active or repressed state of homeotic genes. We have identified a regulatory element in the Ubx gene that responds to Pc-G and trx-G genes. Transposons carrying this element create new binding sites for Pc-G products in the polytene chromosomes. This Pc-G maintenance element (PRE), establishes a repressive complex that keeps enhancers repressed in cells in which they were originally repressed and maintains this state through many cell divisions. The trx-G products stimulate the expression of enhancers in cells in which they were originally active. This mechanism is responsible for the correct regulation of imaginal disc enhancers, which lack themselves antero-posterior positional information. The PRE also causes severe variegation of the mini-white gene present in the transposon, a phenomenon very similar to heterochromatic position-effect variegation. The significance of this mechanism for homeotic gene regulation is discussed.
分节基因提供信号，用于激活和调控果蝇中的同源异型基因，但无法维持所产生的表达模式，因为它们的活性在胚胎发育中途就会停止。这种模式的维持归因于多梳基因家族（Pc-G）和三胸基因家族（trx-G），它们负责同源异型基因活性或抑制状态的持续存在。我们在Ubx基因中鉴定出了一个对Pc-G和trx-G基因有反应的调控元件。携带该元件的转座子在多线染色体中为Pc-G产物创造了新的结合位点。这个Pc-G维持元件（PRE）建立了一个抑制复合物，该复合物使增强子在最初被抑制的细胞中保持抑制状态，并通过许多细胞分裂维持这种状态。trx-G产物刺激增强子在最初活跃的细胞中的表达。这种机制负责对成虫盘增强子进行正确调控，而成虫盘增强子本身缺乏前后位置信息。PRE还会导致转座子中存在的mini-white基因出现严重的斑驳现象，这一现象与异染色质位置效应斑驳非常相似。本文讨论了这种机制对同源异型基因调控的意义。
