DuBois M L, Prescott D M
Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder 80309-0347, USA.
Mol Cell Biol. 1997 Jan;17(1):326-37. doi: 10.1128/MCB.17.1.326.
Germ line micronuclear genes in ciliated protozoa contain two types of interrupting sequences. Some genes contain introns, but internal eliminated segments (IESs) are much more prevalent. IESs are AT-rich DNA segments that separate macronucleus-destined segments (MDSs) in micronuclear genes. All IESs are excised and destroyed when a micronucleus develops into a macronucleus after each cell mating. IESs have no discernible function. Therefore, an investigation of the behavior of IESs in evolution has been undertaken to assess their possible significance. The IESs in the micronuclear gene encoding the beta-subunit of the telomere-binding protein (beta-TP) are not conserved in number, position, sequence, or length during the evolution of four oxytrichid ciliates. In contrast, the scrambled pattern of MDSs and IESs of the micronuclear actin I gene has been conserved during evolution; however, the precise positions, sequences, and lengths of the IESs differ among species, and in some organisms the actin I gene contains an additional IES and MDS. Corresponding IESs in the actin I genes among the different organisms have shifted positions by 1 to 14 bp, presumably by a mutation-shifting mechanism, creating differences in the repeat sequences flanking IESs. Thus, conservation of a particular repeat sequence among species is not required for IES excision. The changes in IES number and position in the beta-TP genes among ciliates are in sharp contrast to the stability of the intron position. Therefore, IESs are volatile, hypermutable elements that are inserted, removed, shifted, and modified continuously in the germ line through evolutionary time.
纤毛原生动物的种系微核基因包含两种类型的中断序列。一些基因含有内含子，但内部消除片段（IESs）更为普遍。IESs是富含AT的DNA片段，它们在微核基因中分隔注定进入大核的片段（MDSs）。在每次细胞交配后，当微核发育成大核时，所有IESs都会被切除并破坏。IESs没有可识别的功能。因此，已经对IESs在进化中的行为进行了研究，以评估它们可能具有的重要性。在四种尖毛虫纤毛虫的进化过程中，编码端粒结合蛋白β亚基（β-TP）的微核基因中的IESs在数量、位置、序列或长度上并不保守。相比之下，微核肌动蛋白I基因的MDSs和IESs的混乱模式在进化过程中得以保留；然而，IESs的精确位置、序列和长度在不同物种之间存在差异，并且在一些生物体中，肌动蛋白I基因还包含一个额外的IES和MDS。不同生物体中肌动蛋白I基因的相应IESs的位置已经移动了1到14个碱基对，推测是通过突变移位机制，从而在IESs侧翼的重复序列中产生差异。因此，IES切除并不需要物种间特定重复序列的保守性。纤毛虫中β-TP基因的IES数量和位置的变化与内含子位置的稳定性形成了鲜明对比。因此，IESs是不稳定的、高度可变的元件，在进化过程中通过种系不断地被插入、去除、移位和修饰。
