Hopmann R, Duncan D, Duncan I
Department of Biology, Washington University, St. Louis, Missouri 63130, USA.
Genetics. 1995 Feb;139(2):815-33. doi: 10.1093/genetics/139.2.815.
The Abdominal-B (Abd-B) gene of the bithorax complex (BX-C) of Drosophila controls the identities of the fifth through seventh abdominal segments and segments in the genitalia (more precisely, parasegments 10-14). Here we focus on iab-5, iab-6 and iab-7, regulatory regions of Abd-B that control expression in the fifth, sixth and seventh abdominal segments (parasegments 10-12). By analysis of partial BX-C deficiencies, we show that these regions are able to promote fifth and sixth abdominal segment identities in the absence of an Abd-B gene in cis. We establish that this ability does not result from cis-regulation of the adjacent abd-A or Ubx genes of the BX-C but rather occurs because the iab-5,6,7 region is able to interact with Abd-B in trans. We demonstrate that this interaction is proximity dependent and is, therefore, a case of what E. B. Lewis has called transvection. Interactions of this type are presumably facilitated by the synapsis of homologues that occurs in somatic cells of Dipterans. Although transvection has been detected in a number of Drosophila genes, transvection of the iab-5,6,7 region is exceptional in two ways. First, interaction in trans with Abd-B does not require that homologues share homologous sequences within, or for some distance to either side of, the BX-C. This is the first case of transvection shown to be independent of local synapsis. A second unusual feature of iab-5,6,7 transvection is that it is remarkably difficult to disrupt by heterozygosity for chromosome rearrangements. The lack of requirement for local synapsis and the tenacity of trans-interaction argue that the iab-5,6,7 region can locate and interact with Abd-B over considerable distance. This is consistent with the normal role of iab-5,6,7, which must act over some 20-60 kb to influence its regulatory target in cis at the Abd-B promoter. Evidence is presented that trans-action of iab-5,6,7 requires, and may be mediated by, the region between distal iab-7 and Abd-B. Also, we show that iab-5,6,7 transvection is independent of the allelic state of zeste, a gene that influences several other cases of transvection. The long-range nature of interactions in trans between iab-5,6,7 and Abd-B suggests that similar interactions could operate effectively in organisms lacking extensive somatic pairing. Transvection may, therefore, be of more general significance than previously suspected.
果蝇双胸复合体（BX-C）的腹部B（Abd-B）基因控制着第五至第七腹节以及生殖器部分节段（更准确地说是副节10 - 14）的特征。在这里，我们聚焦于iab-5、iab-6和iab-7，它们是Abd-B的调控区域，控制着第五、第六和第七腹节（副节10 - 12）中的表达。通过对部分BX-C缺失的分析，我们发现这些区域在顺式作用中能够在没有Abd-B基因的情况下促进第五和第六腹节的特征形成。我们确定这种能力并非源于BX-C相邻的abd-A或Ubx基因的顺式调控，而是因为iab-5、6、7区域能够与Abd-B进行反式相互作用。我们证明这种相互作用依赖于空间 proximity，因此是E. B. 刘易斯所称的异位效应的一个例子。这种类型的相互作用可能是由双翅目体细胞中同源染色体的联会促进的。尽管在许多果蝇基因中都检测到了异位效应，但iab-5、6、7区域的异位效应在两个方面是特殊的。首先，与Abd-B的反式相互作用并不要求同源染色体在BX-C内部或两侧一定距离内具有同源序列。这是首次显示的与局部联会无关的异位效应的例子。iab-5、6、7异位效应的另一个不寻常特征是，它极难因染色体重排的杂合性而被破坏。对局部联会的缺乏需求以及反式相互作用的持久性表明，iab-5、6、7区域能够在相当远的距离定位并与Abd-B相互作用。这与iab-5、6、7的正常作用是一致的，它必须在约20 - 60 kb的范围内起作用以顺式影响其在Abd-B启动子处的调控靶点。有证据表明，iab-5、6、7的反式作用需要并且可能由远端iab-7和Abd-B之间的区域介导。此外，我们表明iab-5、6、7异位效应独立于zeste的等位基因状态，zeste是一个影响其他几个异位效应情况的基因。iab-5、6、7与Abd-B之间反式相互作用的长距离性质表明，类似的相互作用可能在缺乏广泛体细胞配对的生物体中有效发挥作用。因此，异位效应可能比以前怀疑的具有更普遍的意义。
