Margolis J S, Borowsky M L, Steingrímsson E, Shim C W, Lengyel J A, Posakony J W
Department of Biology, University of California San Diego, La Jolla 92093-0366, USA.
Development. 1995 Sep;121(9):3067-77. doi: 10.1242/dev.121.9.3067.
The gap gene hunchback (hb) is required for the formation and segmentation of two regions of the Drosophila embryo, a broad anterior domain and a narrow posterior domain. Accumulation of hb transcript in the posterior of the embryo occurs in two phases, an initial cap covering the terminal 15% of the embryo followed by a stripe at the anterior edge of this region. By in situ hybridization with transcript-specific probes, we show that the cap is composed only of mRNA from the distal transcription initiation site (P1), while the later posterior stripe is composed of mRNA from both the distal and proximal (P2) transcription initiation sites. Using a series of genomic rescue constructs and promoter-lacZ fusion genes, we define a 1.4 kb fragment of the hb upstream region that is both necessary and sufficient for posterior expression. Sequences within this fragment mediate regulation by the terminal gap genes tailless (tll) and a huckebein, which direct the formation of the posterior hb stripe. We show that the tll protein binds in vitro to specific sites within the 1.4 kb posterior enhancer region, providing the first direct evidence for activation of gene expression by tll. We propose a model in which the anterior border of the posterior hb stripe is determined by tll concentration in a manner analogous to the activation of anterior hb expression by bicoid.
间隙基因驼背（hb）对于果蝇胚胎两个区域的形成和分割是必需的，这两个区域分别是一个宽阔的前部区域和一个狭窄的后部区域。hb转录本在胚胎后部的积累分两个阶段进行，最初是覆盖胚胎末端15%的帽状结构，随后是该区域前缘的一条带。通过与转录本特异性探针进行原位杂交，我们发现帽状结构仅由来自远端转录起始位点（P1）的mRNA组成，而后来的后部条带则由来自远端和近端（P2）转录起始位点的mRNA组成。使用一系列基因组拯救构建体和启动子 - lacZ融合基因，我们确定了hb上游区域的一个1.4 kb片段，该片段对于后部表达既是必需的也是充分的。该片段内的序列介导了由末端间隙基因无尾（tll）和huckebein的调控，它们指导后部hb条带的形成。我们表明tll蛋白在体外与1.4 kb后部增强子区域内的特定位点结合，为tll激活基因表达提供了首个直接证据。我们提出了一个模型，其中后部hb条带的前缘由tll浓度决定，其方式类似于由双尾激活前部hb表达。
