The in vivo conformation of the Plastid DNA of Toxoplasma gondii: Implications for replication

The Phylum Apicomplexa comprises thousands of obligate intracellular parasi tes, some of which cause serious disease in man and other animals. Though n ot photosynthetic, some of them, including the malaria parasites (Plasmodiu m spy.) and the causative organism of Toxoplasmosis, Toxoplasma gondii, pos sess a remnant plastid partially determined by a highly derived residual ge nome encoded in 35 kb DNA. The genetic maps of the plastid genomes of these two organisms are extremely similar in nucleotide sequence, gene function and gene order. However, a study using pulsed field gel electrophoresis and electron microscopy has shown that in contrast to the malarial version, on ly a minority of the plastid DNA of Toxoplasma occurs as circular 35 kb mol ecules. The majority consists of a precise oligomeric series of linear tand em arrays of the genome, each oligomer terminating at the same site in the genetic map, i.e. in the centre of a large inverted repeat (IR) which encod es duplicated tRNA and rRNA genes. This overall topology strongly suggests that replication occurs by a rolling circle mechanism initiating at the cen tre of the JR, which is also the site at which the linear tails of the roll ing circles are processed to yield the oligomers. A model is proposed which accounts for the quantitative structure of the molecular population. It is relevant that a somewhat similar structure has been reported for at least three land plant chloroplast genomes. (C) 2001 Academic Press.