The chytrid fungus Spizellomyces punctatus and the amoeboid protozoan Acanthamoeba castellanii share a similar tRNA editing mechanism.

Marie-Josée Laforest, Ingeborg Roewer and B. Franz Lang .

We have identified eight tRNA-like structures in the completely sequenced mitochondrial (mt) DNA of the chytridiomycete Spizellomyces punctatus. The predicted products of these genes have one to three mismatches in the acceptor stem, which, if not edited, would prevent proper folding of the RNA structure and processing of the tRNAs from their precursors. We sequenced the cDNA of 3 tRNAs (lys[uuu]; leu[cua]; tyr[gua]), and discovered that the mismatches are corrected through RNA editing. The editing pattern observed is very similar to the one described for A. castellanii, an amoeboid protozoan.

The existence of an apparently very similar tRNA editing mechanism in mitochondria of a chytridiomycete and an amoeba is puzzling, because there is no indication that these organisms share a relatively recent, common phylogenetic ancestor. In fact, phylogenetic analysis using mitochondrial or nuclear sequence data clearly positions S. punctatus as an early diverging fungus, whereas A. castellanii cannot be placed with certainty, but tends to branch with very early protist lineages.

We have more recent molecular evidence that the same type of tRNA editing also occurs in another lower fungal lineage, the Monoblepharidales. While tRNA editing exists in all three analyzed Monoblepharidales, Harpochytrium#94, Harpochytrium#105 and Monoblepharella sp., the mitochondrial tRNA structures of a member of the Chytridiales, Rhizophydium sp., are all normal, not indicating any need for post-transcriptional modification. Because the Spizellomycetales and the Chytridiales are sister clades (see phylogenetic analysis), to the exclusion of theMonoblepharidales, it is likely that tRNA editing in the Spizellomycetales and Monoblepharidales arose in separate events (as already suggested for Acanthamoeba).

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