AU tested members of genus Plasmodium contain tandemly arrayed, transcribed, extrachromosomal DNA with a unit length of 6.0 kb. This DNA contains two open reading frames with potential to encode cytochrome c oxidase subunit I (coxl) and cytochrome b (cob) as well as fragments of rRNA genes scattered on both strands. At least 10 discrete RNA molecules transcribed during erythrocytic stages of a rodent malarial parasite, Plasmodium yoelii, were recognized by the 6.0-kb DNA probes. The RNA molecules of 1.4 and 1.1 kb were identified as encoding coxi and cob, respectively. Primer extension and RNA sequencing were used to locate and characterize 5' ends of these two RNAs, showing that an identical 12-nucleotide sequence, 5'-TATTTTT TGTTT-3', was present at these positions. This sequence may act as a promoter or as an RNA processing signal. A stem-loop structure signifying a possible transcription termination was present at the end of the coxi open reading frame. At least six discrete RNA molecules of less than 250 nucleotides were recognized by different fractions of the 6.0-kb DNA. The largest of these, 200 nucleotides, was also characterized by primer extension and RNA sequencing. This molecule had a high homology to portions of the large-subunit rRNA domains IV and V. Other, small RNA molecules were recognized by regions of the 6.0-kb DNA that had homology to the highly conserved peptidyltransferase domain of large-subunit rRNA. These results show that tlte unusual compactly organized mitochondrionlike DNA of malarial parasites is transcribed in a complex attern.Malarial parasites, members of genus Plasmodium, belong to phylum Apicomplexa of kingdom Protista. Organisms within this phylum include many important pathogens, such as species of the genera Toxoplasma, Babesia, and Eimeria, in addition to Plasmodium species, all of which are intracellular parasites. They possess a complex array of organelles, some of which may assist in invasion of host cells; others have unknown functions. Among the organelles of Plasmodium species are mitochondria, whose contributions to the parasite physiology are not entirely clear. Because mammalian Plasmodium species rely mainly on glycolysis for energy generation and lack many citric acid cycle enzymes (22), mitochondria have been thought to act primarily as electron sink for dihydro-orotate dehydrogenase during pyrimidine biosynthesis (17). And yet, observations with rhodamine 123 staining (11,24) and studies with mitochondrial inhibitors (4, 16) give a hint of organelles that may play a wider role in parasite biochemistry.Until recently, not much was known about the organelle DNA of malarial parasites. A circular DNA molecule of about 35 kb was observed by electron microscopy and assumed to be the mitochondrial genome (12,18,28). This assumption appeared to be substantiated when genes encoding rRNA sequences resembling those of procaryotes and mitochondria were found within the 35-kb circular DNA of Plasmodium falciparum (15). However, through DNA sequencing of a 6.0-kb tandemly...
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