Origin, evolution, and mechanism of 5′ tRNA editing in chytridiomycete fungi
- 1Département de Biochimie, Université de Montréal, Succursale Centre-Ville, Montréal, Québec H3C 3J7, Canada
- 2Institut de Biologie Moléculaire des Plantes du Centre National de la Recherche Scientifique, 67084 Strasbourg Cedex, France
- 3Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada
- 4Program in Evolutionary Biology, Canadian Institute for Advanced Research, Toronto, Ontario M5G1Z8; Canada
Abstract
5′ tRNA editing has been demonstrated to occur in the mitochondria of the distantly related rhizopod amoeba Acanthamoeba castellanii and the chytridiomycete fungus Spizellomyces punctatus. In these organisms, canonical tRNA structures are restored by removing mismatched nucleotides at the first three 5′ positions and replacing them with nucleotides capable of forming Watson–Crick base pairs with their 3′ counterparts. This form of editing seems likely to occur in members of Amoebozoa other than A. castellanii, as well as in members of Heterolobosea. Evidence for 5′ tRNA editing has not been found to date, however, in any other fungus including the deeply branching chytridiomycete Allomyces macrogynus. We predicted that a similar form of tRNA editing would occur in members of the chytridiomycete order Monoblepharidales based on the analysis of complete mitochondrial tRNA complements. This prediction was confirmed by analysis of tRNA sequences using a tRNA circularization/ RT-PCR-based approach. The presence of partially and completely unedited tRNAs in members of the Monoblepharidales suggests the involvement of a 5′-to-3′ exonuclease rather than an endonuclease in removing the three 5′ nucleotides from a tRNA substrate. Surprisingly, analysis of the mtDNA of the chytridiomycete Rhizophydium brooksianum, which branches as a sister group to S. punctatus in molecular phylogenies, did not suggest the presence of editing. This prediction was also confirmed experimentally. The absence of tRNA editing in R. brooksianum raises the possibility that 5′ tRNA editing may have evolved twice independently within Chytridiomycota, once in the lineage leading to S. punctatus and once in the lineage leading to the Monoblepharidales.
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Footnotes
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Article published online ahead of print. Article and publication date are at http://www.rnajournal.org/cgi/doi/10.1261/rna.7330504.
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- Accepted April 30, 2004.
- Received March 8, 2004.
- Copyright 2004 by RNA Society
