THE EXTRACHROMOSOMAL DNAS OF APICOMPLEXAN PARASITES

Molecular analyses in recent years have begun to elucidate the identit y and role of two extrachromosomal DNAs found in apicomplexan parasite s. One of these is a small tandemly repeated DNA that encodes three cl assical mitochondrial protein coding genes, attesting to its identity. This molecule also encodes mitochondrial rRNAs as small fragments in scattered locations. Despite their unusual nature, evidence suggests t hat these rRNAs are functional. They offer an opportunity to evaluate structure-function correlations in the absence of much of the more var iable sequences found in other rRNAs. The second extrachromosomal DNA has characteristics reminiscent of chloroplast DNAs and thus points to an unexpected ancestry for the apicomplexans. Both DNAs are inherited maternally, as is usual for organelle DNAs, but their subcellular loc ations have not been demonstrated unequivocally. Although the majority of studies with these two DNAs thus far have been with Plasmodium spe cies, evidence from other apicomplexans suggests that these unusual or ganelle genomes are common to the phylum.