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Mitochondrial tRNAs in the lower fungus Spizellomyces punctatus: tRNA editing and UAG 'stop' codons recognized as
M J Laforest1, I Roewer, B F Lang
1Département de Biochimie, Université de Montréal, CP 6128, succ. Centre Ville, Montréal , Québec H3C 3J7, Canada.
Abstract:
The mitochondrial DNA of the chytridiomycete fungus Spizellomyces punctatusen codes only eight tRNAs, although a minimal set of 24-25 tRNAs is normally found in fungi. One of these tRNAs has a CAU anticodon and is structurally related to leucine tRNAs, which would permit the translation of the UAG 'stop' codons that occur in most of its protein genes. The predicted structures of all S. punctatus tRNAs have the common feature of containing one to three mis-pairings in the first three positions of their acceptor stems. Such mis-pairing is expected to impair proper folding and processing of tRNAs from their precursors. Five of these eight RNAs were shown to be edited at the RNA level, in the 5'portion of the molecules. These changes include both pyrimidine to purine and A to G substitutions that restore normal pairing in the acceptor stem. Editing was not found at other positions of the tRNAs, or in the mitochondrial mRNAs of S. punctatus. While tRNA editing has not been observed in other fungi, the editing pattern inS.punctatus is virtually identical to that described in the amoeboid protozoan Acanthamoeba castellanii. If this type of mitochondrial tRNA editing has originated from their common ancestor, one has to assume that it was independently lost in plants, animals and in most fungi. Alternatively, editing might have evolved independently, or the genes coding for the components of the editing machinery were laterally transferred.
Insights
Mitochondria in the fungus Spizellomyces punctatus use RNA editing to fix transfer RNA (tRNA) misfolding, a process not seen in other fungi but similar to that in Acanthamoeba. This suggests ancient origins or independent evolution of tRNA editing.
Area of Science:
- Mitochondrial genetics
- Molecular evolution
- RNA biology
Background:
- Fungal mitochondrial genomes typically encode 24-25 tRNAs, essential for protein synthesis.
- Spizellomyces punctatus mitochondrial DNA encodes only eight tRNAs, with one potentially translating stop codons.
- Predicted tRNA structures in S. punctatus show mis-pairings in acceptor stems, potentially hindering function.
Purpose of the Study:
- To investigate the structure and processing of mitochondrial tRNAs in Spizellomyces punctatus.
- To determine if RNA editing occurs in S. punctatus mitochondrial tRNAs and mRNAs.
- To compare the observed tRNA editing patterns with those in other organisms.
Main Methods:
- Analysis of mitochondrial DNA and predicted tRNA structures.
- RNA sequencing and analysis to detect post-transcriptional modifications.
- Comparative analysis of tRNA editing patterns.
Main Results:
- Five of the eight S. punctatus mitochondrial tRNAs undergo RNA editing in their 5' region.
- Editing events, including pyrimidine to purine and A to G substitutions, correct acceptor stem mis-pairings.
- No editing was detected in other tRNA regions or mitochondrial mRNAs.
- The observed tRNA editing pattern is highly similar to that in Acanthamoeba castellanii.
Conclusions:
- Mitochondrial tRNA editing in S. punctatus is a crucial mechanism for functional tRNA maturation.
- The conserved editing pattern suggests either a common ancestral origin followed by loss in other lineages or independent evolution/lateral gene transfer.
- This finding challenges the understanding of tRNA evolution and editing mechanisms across eukaryotes.