A Circular Code in the Protein Coding Genes of Mitochondria

A new maximal circular code X0(MIT) with two permutated maximal circular codes X1(MIT) and X2(MIT) is identified in the protein coding genes of mitochondria. The three subsets of 20 trinucleotides X0(MIT)={ACA, ACC, ATA, ATC, CTA, CTC, GAA, GAC, GAT, GCA, GCC, GCT, GGA, GGC, GGT, GTA, GTC, GTT, TTA, TTC}, X1(MIT) and X2(MIT) are in frame 0 (reading frame), 1 and 2 respectively. X1(MIT) and X2(MIT) are deduced by one and two circular permutations of X0(MIT) respectively. The code X0(MIT) has four important properties: a length of the minimal window to automatically retrieve frame 0 which is equal to five nucleotides; an occurrence probability equal to 6.3×10−5; a low frequency (12% in average) of misplaced trinucleotides in the shifted frames; and an occurrence of four types of nucleotides in the first and second trinucleotide sites but no nucleotide G in the third trinucleotide site. Several biological consequences are presented in the Discussion.