A 7.1 KB LINEAR DNA MOLECULE OF THEILERIA-PARVA HAS SCRAMBLED RDNA SEQUENCES AND OPEN READING FRAMES FOR MITOCHONDRIALLY ENCODED PROTEINS

Theileria parva, an intralymphocytic protozoan parasite of cattle, con tains a linear 7.1 kb DNA element with terminal inverted repeat sequen ces. The molecule is transcribed into low molecular weight RNA, and bo th DNA strands encode short stretches of unique sequences, usually <10 0 nucleotides, which are similar to large (LSU) or small (SSU) ribosom al subunit RNA. Phylogenetically conserved conformational rRNA domains were assembled from the discontinuous rDNA sequences using comparativ e secondary structure modelling. For example, a minimum of four predic ted sequences, two derived from each DNA strand, is required to assemb le domain V of LSU rRNA which participates in peptidyl transferase act ivity. The discontinuities in the identified rRNA domains fall within regions of no known functional significance. Hence, it is likely that the element encodes fragmented rDNA genes and the mature rRNA is uncon ventional, consisting of several fragments of RNA, primarily held toge ther by intermolecular and intramolecular base pairing. The element al so has ORFs for components of the last two mitochondrial electron tran sport enzyme complexes. The structure of the parasite DNA element, its protein coding capacity and scrambled rDNA gene sequences, are remini scent of the mitochondrial genome of Chlamydomonas reinhardtii. We pro pose that the 7.1 kb element is equivalent to the mitochondrial DNA of T.parva, although a number of its features are unusual for this famil y of extrachromosomal DNA molecules.