True Mitochondrial tRNA Punctuation and Initiation Using Overlapping Stop and Start Codons at Specific and Conserved Positions
Résumé
In all the taxa and genomic systems, numerous trn genes (specifying tRNA) exhibit at specific
conserved positions nucleotide triplets corresponding to stop codons (TAG/TAA).
Similarly, relatively high frequencies of start codons (ATG/ATA) occur in fungi/metazoan
mitochondrial-trn genes. The last nucleotide of these triplets is the first involved in the 5′-Dor
5′-T-stem, respectively. Their frequencies are tRNA species dependent. The products of
these genes which bear one or two types of these codons are called ss-tRNAs (for stop/start).
Metazoan mt-genomes are generally very compact, and many same strand overlapping
sequences may simultaneously code for tRNAs and mRNAs. However, this study suggests
that overlaps are not a direct mechanism to substantially reduce genome size. For proteinencoding
genes, occulting possible overlaps, there are only alternative start codons and/or
truncated stop codons, but the first putative in-frame standard initiation codon or complete
stop codon is in the upstream or downstream overlapping ss-trn sequences, respectively.
Even if, to date, experimental data are missing, stress signals might regulate producing
extended or not proteins. Finally, possible implications of tRNA/mRNA hybrid molecules
in the “RNA world” to “RNA/protein world” transition will be discussed