Complex transcription from the extrachromosomal DNA encoding mitochondrial functions of Plasmodium yoelii

Abstract

All 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 (cox1) 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 cox1 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 cox1 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 the unusual compactly organized mitochondrionlike DNA of malarial parasites is transcribed in a complex pattern.