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By using a Trypanosoma brucei alpha-tubulin cDNA probe under reduced stringency hybridization conditions, we have isolated two genomic clones that constitute portions of alpha-tubulin genes of the rodent malarial parasite Plasmodium yoelii. P. yoelii has two alpha-tubulin genes, the 3' portions of which were present in the two clones, Py alpha T1 and Py alpha T2, containing 1.3 kb and 6.6 kb EcoRI fragments respectively. The 1358 bp Py alpha T1 clone was completely sequenced and found to contain 591 nucleotides of uninterrupted coding sequence with a strong bias for AT-rich codons, starting with codon 254 of a consensus alpha-tubulin sequence. Numerous attempts to clone 5' portions of these genes were unsuccessful. A single mRNA of 2.3 kb was recognized by both the clones in the erythrocytic stages of P. yoelii. A probe constituting the untranslated sequences of Py alpha T1 also recognized this RNA but failed to hybridize with Py alpha T2 sequences, indicating that the gene represented by the Py alpha T1 clone was expressed during the erythrocytic stages. The deduced amino acid sequence of the Py alpha T1 gene terminates in Tyr-Glu instead of Glu-Tyr observed in alpha-tubulins of almost all other organisms. The difference observed may have implications for alpha-tubulin metabolism in malarial parasites.

Erythrocytic stages of mammalian malarial parasites contain acristate mitochondria whose functions are not well understood. Moreover, little is known about the genome of these organelles. We have previously reported that all species of malarial parasites examined contain highly conserved, tandemly arrayed DNA with a unit length of about 6.0 kb that is transcribed into discrete RNA molecules in erythrocytic stages. We now report the complete DNA sequence of the 5984-bp repeating unit of Plasmodium yoelii, a rodent parasite. Two slightly overlapping regions transcribed into large RNA molecules were found to have significant DNA and protein sequence similarity with mitochondrion-coded proteins, cytochrome c oxidase subunit I and cytochrome b. Significant sequence similarity with other mitochondrial protein genes could not be detected. Ribosomal RNA (rRNA)-like genes were not detected in this sequence either. However, two regions, 82 and 50 nucleotides long, specified by different strands, were found to have extensive similarity with the highly conserved central loop of the peptidyl transferase domain of the large rRNA of Escherichia coli, mitochondria, and chloroplasts. Compensatory nucleotide substitutions were present in these regions, so that the predicted secondary structure was not affected. Functional utilization of these regions, if it exists, could argue for a trans-associative origin of rRNA. In organization, size and sequence, the tandem arrays of 6.0 kb malarial DNA appear to be a very unusual form of mitochondrial DNA.