Tetrahymena pyriformis

Mitochondrial genome organization, gene content, genetic code

The linear mitochondrial genome of the ciliate protozoon, Tetrahymena pyriformis, comprises 47,172 bp (78.7% A+T), excluding telomeric sequences (identical tandem arrays of 31-bp repeat unit at each end of the mitochondrial genome). In addition to genes encoding the previously described bipartite small subunit (rns_a, rns_b) and large subunit (rnl_a, rnl_b) rRNAs, the T. pyriformis mitochondrial genome contains 21 protein-coding genes that are clearly homologous to genes of defined function in other mtDNAs. These include nad1,2,3,4,5,7,9,10, cob, cox1,2, atp9, rps3,12,13,14,19, rpl2,14,16, and ccl1 (= yejR), a gene that specifies a component of a cytochrome c biogenesis pathway.

In addition, T. pyriformis mtDNA contains 22 ORFs of unknown function larger than 60 codons, ranging in size from 71 to 1380 codons. Thirteen of these ORFs ('ciliate- specific') are also found in Paramecium aurelia mtDNA, whereas the remaining 9 appear to be unique to T. pyriformis. However, 6 of the latter ORFs are positionally equivalent and of similar size in the two mitochondrial genomes, suggesting they may also be ciliate-specific homologs, even though this is not evident from comparisons of the derived amino acid sequences.

Only 7 tRNA genes (trnY,L,F,H,W,E,M) are found in T. pyriformis mtDNA, confirming a long-standing proposal that most T. pyriformis mitochondrial tRNAs must be imported from the cytosol. The mitochondrial translation code is non-standard (UGA = Trp), and codons other than AUG (all of which are variants of AUG) appear to be used frequently to initiate translation.

Novel features of organization and expression of T. pyriformis mtDNA include not only split and rearranged large subunit rRNA genes, but a split nad1 gene (encoding subunit 1 of NADH dehydrogenase of respiratory complex I), the two segments of which are located on and transcribed from opposite mtDNA strands. Gene content and arrangement are very similar in T. pyriformis and P. aurelia mtDNAs, the two genomes differing by a limited number of duplication, inversion and rearrangement events.

Advocate of this sequencing project: M. W. Gray
Collaborators: M. N. Schnare, S. J. Greenwood, T. Littlejohn, G. Burger
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