Herrick G, Hunter D, Williams K, Kotter K
Department of Cellular, Viral and Molecular Biology, University of Utah School of Medicine, Salt Lake City 84132.
Genes Dev. 1987 Dec;1(10):1047-58. doi: 10.1101/gad.1.10.1047.
Macronuclear chromosomes in Oxytricha fallax, a hypotrichous ciliate, are very short. They often belong to small families of cross-hybridizing chromosomes of two or three different sizes. For example, the 81-MAC family consists of three sizes of macronuclear chromosomes (4.9, 2.9, and 1.6 kbp) (Cartinhour and Herrick 1984). We show that the family actually consists of two closely related sets of three each and that the two sets are independently created by alternative processing of two separate precursor (micronuclear) versions. Chromosomes of a set share a common 1.6-kbp region, which contains a transcribed gene coding for a 25-kD protein. Different-sized macronuclear chromosomes of a set result from alternative choices of positions for telomere formation. All six members of the family are reproducibly generated in each developing macronucleus, and their copy numbers are stably maintained during vegetative replication of the macronucleus (Herrick et al. 1987). Here we argue for the existence of three distinct copy control elements in the 81-MAC family chromosomes. A model is discussed in which, following polytenization of the micronuclear chromosomes, different chromatids are processed differently, and, subsequently, replication-competent macronuclear chromosome products are amplified under the influence of the vegetative copy control elements.
伪足纤毛虫类的扇形游仆虫中的大核染色体非常短。它们通常属于由两三种不同大小的交叉杂交染色体组成的小家族。例如，81-MAC家族由三种大小的大核染色体组成（4.9、2.9和1.6千碱基对）（卡廷胡尔和赫里克，1984年）。我们发现，这个家族实际上由两组紧密相关的染色体组成，每组各有三条，并且这两组染色体是由两个独立的前体（微核）版本的交替加工独立产生的。一组染色体共享一个1.6千碱基对的共同区域，该区域包含一个编码25-kD蛋白质的转录基因。一组中不同大小的大核染色体是由端粒形成位置的不同选择导致的。这个家族的所有六个成员在每个发育中的大核中都能可重复地产生，并且它们的拷贝数在大核的营养复制过程中稳定维持（赫里克等人，1987年）。在这里，我们论证了81-MAC家族染色体中存在三种不同的拷贝控制元件。我们讨论了一个模型，在这个模型中，微核染色体多线化后，不同的染色单体被不同地加工，随后，有复制能力的大核染色体产物在营养拷贝控制元件的影响下被扩增。
