Shapiro T A, Englund P T
Department of Medicine, Johns Hopkins School of Medicine, Baltimore, Maryland 21205, USA.
Annu Rev Microbiol. 1995;49:117-43. doi: 10.1146/annurev.mi.49.100195.001001.
The mitochondrial DNA of trypanosomatid protozoa, termed kinetoplast DNA (kDNA), is unique in its structure, function, and mode of replication. kDNA is a massive network, composed of thousands of topologically interlocked DNA circles, which resembles the chain mail of medieval armor. Each cell contains one network condensed into a disk-shaped structure within the matrix of its single mitochondrion. The kDNA circles are of two types, maxicircles present in a few dozen copies and minicircles present in several thousand copies. The maxicircles, which encode ribosomal RNAs and a few mitochondrial proteins, are similar in structure and genetic function to the mitochondrial DNA of other eukaryotes. Many maxicircle transcripts undergo editing, a remarkable process involving the insertion or deletion of uridine residues at specific sites. The minicircles encode small guide RNAs that control the specificity of editing. During kDNA replication, covalently closed minicircles are released from the network by a topoisomerase II. The free minicircles replicate as theta-structures within one of two complexes of replication proteins that are positioned on opposite sides of the kinetoplast disk. The progeny minicircles, which contain nicks or gaps, are attached to the network periphery. Maxicircles also replicate as theta-structures, but they remain linked to the network. As replication proceeds, the number of minicircles and maxicircles increases. When the network has doubled in size, all of the minicircle nicks and gaps are repaired, and the network splits in two. The two progeny networks then segregate into the daughter cells.
锥虫类原生动物的线粒体DNA，称为动质体DNA（kDNA），在其结构、功能和复制模式方面独具特色。kDNA是一个庞大的网络，由数千个拓扑互锁的DNA环组成，类似于中世纪盔甲的链甲。每个细胞含有一个网络，该网络在其单个线粒体的基质中浓缩成盘状结构。kDNA环有两种类型，几十份拷贝的大环状DNA和几千份拷贝的小环状DNA。编码核糖体RNA和一些线粒体蛋白的大环状DNA在结构和遗传功能上与其他真核生物的线粒体DNA相似。许多大环状DNA转录本会经历编辑，这是一个在特定位点插入或删除尿苷残基的显著过程。小环状DNA编码控制编辑特异性的小向导RNA。在kDNA复制过程中，共价闭合的小环状DNA通过拓扑异构酶II从网络中释放出来。游离的小环状DNA在位于动质体盘两侧的两个复制蛋白复合物之一内以θ结构进行复制。含有切口或缺口的子代小环状DNA附着在网络外围。大环状DNA也以θ结构进行复制，但它们仍与网络相连。随着复制的进行，小环状DNA和大环状DNA的数量增加。当网络大小翻倍时，所有小环状DNA的切口和缺口都被修复，网络一分为二。然后两个子代网络分离到子细胞中。
