Brunk C F, Navas P A
Biology Department and Molecular Biology Institute, University of California, Los Angeles.
Dev Genet. 1992;13(2):111-7. doi: 10.1002/dvg.1020130204.
In Tetrahymena, the DNA of the macronucleus exists as very large (100 to 4,000-kb) linear molecules that are randomly partitioned to the daughter cells during cell division. This genetic system leads directly to an assortment of alleles such that all loci become homozygous during vegetative growth. Apparently, there is a copy number control mechanism operative that adjusts the number of each macronuclear DNA molecule so that macronuclear DNA molecules (with their loci) are not lost and aneuploid death is a rare event. In comparing Southern analyses of the DNA from various species of Tetrahymena using histone H4 genes as a probe, we find different band intensities in many species. These differences in band intensities primarily reflect differences in the copy number of macronuclear DNA molecules. The variation in copy number of macronuclear DNA molecules in some species is greater than an order of magnitude. These observations are consistent with a developmental control mechanism that operates by increasing the macronuclear copy number of specific DNA molecules (and the genes located on these molecules) to provide the relatively high gene copy number required for highly expressed proteins.
在四膜虫中，大核DNA以非常大的（100至4000千碱基对）线性分子形式存在，这些分子在细胞分裂期间随机分配到子细胞中。这种遗传系统直接导致等位基因的分类，使得在营养生长期间所有基因座都变为纯合子。显然，存在一种起作用的拷贝数控制机制，可调节每个大核DNA分子的数量，以使大核DNA分子（及其基因座）不会丢失，并且非整倍体死亡是罕见事件。在用组蛋白H4基因作为探针比较不同四膜虫物种的DNA的Southern分析时，我们发现在许多物种中带强度不同。这些带强度的差异主要反映了大核DNA分子拷贝数的差异。某些物种中大核DNA分子拷贝数的变化大于一个数量级。这些观察结果与一种发育控制机制一致，该机制通过增加特定DNA分子（以及位于这些分子上的基因）的大核拷贝数来提供高表达蛋白质所需的相对较高的基因拷贝数。
