Replication Mechanism of Mitochondrial DNA from Paramecium aurelia: Sequence of the Cross-linked Origin

Abstract

With all the elegant genetic and biochemical information on fungal, plant, and mammalian mitochondria presented in this volume, one might ask, Why study Paramecium? In many respects, Paramecium is a choice organism. Its size (150 μm × 100 μm) makes it easily amenable to microinjection, and it was in this organism that transformation to antibiotic resistance by the direct injection of purified mitochondria was first demonstrated (see Beale and Knowles 1976). Moreover, by examination of mtDNAs from donor, recipient, and transformed cells, we were able to demonstrate that it was indeed the donor DNA that brought about this transformation (Cummings et al. 1976). In addition, Paramecium mtDNA may represent an intermediate case between yeast and mammalian cells: GC content, 37%; size, 14 μm; lack of intervening sequences in rDNA; and a separation of rRNA genes of about 10 kbp (Maki and Cummings 1977; Cummings et al. 1980). But most importantly, mtDNA from Paramecium is a linear molecule (Goddard and Cummings 1975), making it unlike other mtDNAs. Only in Tetrahymena, another ciliate, has linear mtDNA also been demonstrated (Suyama and Miura 1968).

On the basis of electron microscopy studies, the replication scheme of the linear mtDNA from Paramecium can be depicted as in Figure 1. Partial denaturation studies showed that there is an AT-rich “bubble” at one end of the monomer molecule, and this is shown here as a cross-link. Replication proceeds by means of “lariat” intermediates, terminating in a dimer-length linear molecule. Growth in the presence of ethidium bromide caused...