Translation of mitochondrial swinger RNAs according to tri-, tetra- and pentacodons.

Transcriptomes and proteomes include RNA and protein fragments not matching regular transcription/translation. Some 'non-canonical' mitochondrial transcripts match mitogenomes after assuming one among 23 systematic exchanges between nucleotides, producing swinger RNAs (nine symmetric, X↔Y, example C↔T; 14 asymmetric, X→Y→Z→X, example A→T→G→A) in GenBank's EST database. Here, reanalyzes of (a) public human mitochondrial transcriptome data (Illumina: RNA-seq) allowed to detect mitochondrial swinger RNAs for all 23 exchanges and (b) independent public human mitochondrial trypsinized proteomic mass spectrometry data allowed to detect peptides predicted from translation of parts of swinger-transformed mitogenomes covered by detected swinger reads. RNA-seq and previous EST swinger transcript data converge. Swinger RNA translation frequently inserts various amino acids at stop codons. Swinger RNA-peptide associations exist also for peptides matching systematically frameshifting translation, peptides entirely coded by tetra- and pentacodons (regular codons expanded by silent mononucleotides at 4th, and silent dinucleotides at 4th and 5th position(s), respectively). Swinger peptides differ from regular mitochondrial proteins: not membrane embedded, reflect warmer, anaerobic, low resource conditions, reminding a free-living ancestor. Tetra- and pentacoded peptides associate with low, high GC contents, respectively, suggesting expanded codon translations associate with thermic stresses. Results confirm experimentally predicted swinger, tetra- and pentacoded mitochondrial peptides, increasing mitogenomic coding density.