40 Kinetoplast DNA Replication

The order Kinetoplastida is composed of several families of flagellated protozoa which are among the most ancient of the eukaryotes. The most prominent members of this order are parasites that are responsible for major tropical diseases. These parasites include Trypanosoma brucei, the African trypanosome; Trypanosoma cruzi, the South American trypanosome; and several species of Leishmania. Other kinetoplastids include Crithidia fasciculata, an insect parasite, and species of Phytomonas, which are parasites of plants. All of the kinetoplastids possess a distinctive disk-shaped structure, known as the kinetoplast, within their single mitochondrion. The kinetoplast is composed of the cell’s mitochondrial DNA, known as kinetoplast DNA (kDNA). kDNA has a structure unlike that of any other DNA in nature. It consists of two types of circular DNA molecules, maxicircles and minicircles, which are topologically interlocked into a single, massive network. Figure 1 shows an electron micrograph of part of a kDNA network from C. fasciculata (for reviews on kDNA, see Ray 1987; Simpson 1987; Stuart and Feagin 1992; Shapiro and Englund 1995). There is substantial information on the genetic function of maxicircles and minicircles. Maxicircles resemble conventional mitochondrial DNA from mammals or yeast, in that they encode rRNAs and proteins involved in mitochondrial energy transduction (such as cytochrome oxidase subunits). A unique feature of maxicircles is that many of their transcripts undergo RNA editing, a process by which uridine residues are inserted or deleted at specific sites (for review, see Hajduk et al. 1993). In some cases, editing occurs on a massive scale, with...