Pan D, Valentine S A, Courey A J
Department of Chemistry and Biochemistry, University of California, Los Angeles 90024-1569.
Mech Dev. 1994 Apr;46(1):41-53. doi: 10.1016/0925-4773(94)90036-1.
The pivotal role of twist in mesoderm determination in the Drosophila embryo depends upon two processes--the transcriptional activation of twist in the ventrally located mesodermal anlage and the regulation of downstream gene expression by the twist transcription factor. To elucidate the molecular mechanisms involved in these processes, we have compared both the coding and regulatory regions of the twist genes from Drosophila melanogaster and Drosophila virilis. Within the coding region, the basic-helix-loop-helix DNA binding and dimerization motif is highly conserved, consistent with the functional importance of this domain. A comparison of the transcriptional regulatory regions reveals a high degree of conservation in the more distal of the two ventral activator regions that have been mapped in the twist 5' flanking region. On the other hand, the more proximal ventral activator region is absent at the corresponding position in the D. virilis twist gene. Instead, there is a region in the second intron of the D. virilis gene that resembles the proximal element of the D. melanogaster gene, in that it consists of little more than a series of whole and half binding sites for the dorsal morphogen. In transformation experiments, the intronic D. virilis element directs an expression pattern that is indistinguishable from that directed by the D. melanogaster proximal VAR. Thus, the twi genes from these two species appear to have evolved enhancer elements with very similar structural and functional properties. These findings suggest that apparently redundant spatially regulated enhancer elements may each play essential roles in fine tuning the level and/or pattern of gene expression.
扭转蛋白在果蝇胚胎中胚层决定中的关键作用取决于两个过程——位于腹侧的中胚层原基中扭转蛋白的转录激活以及扭转转录因子对下游基因表达的调控。为了阐明这些过程中涉及的分子机制，我们比较了黑腹果蝇和粗壮果蝇扭转基因的编码区和调控区。在编码区内，碱性螺旋-环-螺旋DNA结合和二聚化基序高度保守，这与该结构域的功能重要性一致。对转录调控区的比较显示，在扭转基因5'侧翼区已定位的两个腹侧激活区中，更靠远端的区域具有高度保守性。另一方面，在粗壮果蝇扭转基因的相应位置不存在更靠近近端的腹侧激活区。相反，在粗壮果蝇基因的第二个内含子中有一个区域类似于黑腹果蝇基因的近端元件，因为它几乎仅由一系列背侧形态发生素的完整和半结合位点组成。在转化实验中，内含子中的粗壮果蝇元件指导的表达模式与黑腹果蝇近端VAR指导的表达模式无法区分。因此，这两个物种的扭转基因似乎进化出了具有非常相似结构和功能特性的增强子元件。这些发现表明，明显冗余的空间调控增强子元件可能各自在微调基因表达水平和/或模式中发挥重要作用。
