Molecular Genetics of the Drosophila Triplo-lethal Locus

基因剂量的改变通常对生物体的发育和/或生存能力有深远的影响，但对基因剂量效应的分子基础知之甚少。果蝇三倍致死位点（Tpl）是了解基因剂量效应的一个独特而有趣的模型。Tpl是已知的唯一一种在非两个拷贝中存在时是致命的基因位点。具有0、1、3或4个Tpl拷贝的二倍体动物在胚胎晚期或早期一龄幼虫时死亡。Tpl也可能具有不寻常的遗传结构。虽然Tpl的重复和缺陷很容易被分离出来，但消除Tpl功能的点突变从未被发现，这表明Tpl可能包含冗余信息，或者它可能编码非翻译RNA分子。最近的数据表明，Tpl功能所必需的DNA已经被克隆。该区域将通过标准分子技术进行分析，包括cDNA克隆和测序以确定转录单位。候选基因将用于果蝇转化实验中鉴定Tpl。在不久的将来，伯克利果蝇基因组计划将对这一基因组区域进行测序，这将极大地帮助分析这一新的遗传位点。Tpl抑制子位点Su（Tpl）的克隆和测序也将进行。Su（Tpl）突变体胚胎在三次Tpl剂量作用下仍能存活，这表明这两个基因之间存在相互作用。Su（Tpl）的基因组克隆已经获得，cDNA克隆将被分离和分析。了解Su（Tpl）将有助于更好地理解为什么Tpl对剂量如此敏感以及致死率的基础。Tpl有两个特点使其新颖。其一是它对剂量的极度敏感。另一个是它对突变的抵抗力，这表明它有一种不寻常的结构。这两种因素都使其研究更具挑战性，但这类基因在细胞中可能并不罕见。这可能只是因为它们不容易被发现。非蛋白质编码基因也不容易通过DNA序列分析来鉴定。了解Tpl可能会导致发现更多这种类型的基因，并进一步了解基因剂量的重要性。

Christensen9808209Alterations in gene dosage often have profound effects on the development and/or viability of organisms, yet little is known about the molecular basis of gene dosage effects. The Triplo-lethal locus of Drosophila (Tpl) is a unique and interesting model for understanding gene dosage effects. Tpl is the only genetic locus known that is lethal when present in anything other than two copies. Diploid animals with either zero, one, three or four copies of Tpl die as late embryos or early first-instar larvae. Tpl may also have an unusual genetic structure. While duplications and deficiencies of Tpl are readily isolated, point mutations that eliminate Tpl's function have never been found, suggesting that Tpl may consist of redundant information or it may code for a non-translated RNA molecule. Recent data suggest that the DNA that is necessary for Tpl function has been cloned. This region will be analyzed by standard molecular techniques, including cDNA cloning and sequencing to identify transcription units. Candidate genes will be used in Drosophila transformation experiments to identify Tpl. This region of the genome will be sequenced in the near future by the Berkeley Drosophila Genome Project, which will greatly aid the analysis of this novel genetic locus. Cloning and sequencing the Suppressor-of Tpl locus, Su(Tpl), will also be done. Su(Tpl) mutant embryos survive in spite of having three doses of Tpl, which shows an interaction of these two genes. Genomic clones of Su(Tpl) have already been obtained and cDNA clones will be isolated and analyzed. Understanding Su(Tpl) will lead to greater understanding of why Tpl is so dosage sensitive and the basis for the lethality.Tpl has two features that make it novel. One is its extreme dosage sensitivity. The other is its resistance to mutagenesis, suggesting an unusual structure. Both make its study especially challenging, but such genes may not be rare in the cell. It may only be that they are not easily discovered. Non-protein-coding genes will also not easily be identified via DNA sequence analysis. Understanding Tpl may lead to the discovery of many more genes of this type, and a further understanding of the importance of gene dosage.