Codon-Reverse Codon Symmetry

A reverse codon of any codon XYZ is defined as ZYX, where X,Y,Z can be any base. The arrows in table represent pairs of “codon - reverse codons”. For instance, the reverse codon of CCU (Pro) is UCC (Ser). There exist 15 different amino acids in the rows 000, 101, 010 and 111 where the codon is reverse to itself, e.g. Lys (AAA), Tyr (UAU).  Considering codons and their reverse codons we report three observations.

1. Table  can be divided into four blocks (codon - reverse codon groups) of the same size, for instance the upper left block with Pro (P), Ser (S), Ala (A) and Thr (T). Each block shows the same arrow pattern. All strongly evolutionary conserved groups of amino acids (Thompson et al., 1994) are subsets of exactly one codon - reverse codon group, e.g. the MILV amino acids belong to the upper right block in the table. The other conserved strong groups belonging to one block are STA, NEQK, NHQK, NDEQ, QHRK, MILF, HY. The only exception is FYW.

2. We studied all known tRNA genes of 104 different organisms (Sprinzl et al., 1999). It is known that the STOP codons do not have any tRNA. We found that there are also no tRNA genes containing anticodons reverse to the STOP anticodons (ACT, ATC and ATT in Table 3). This is true for archaea (16), bacteria (81) and most eukaryotes (7). The only exception is H. sapiens, possessing one tRNA^Asn gene with the anticodon ATT, but humans also have three different possible suppressor tRNA genes (Lowe and Eddy, 1997).

3. There are some codon - reverse codon pairs where there only exist tRNAs for one codon, but no tRNA for the reverse codon, e.g. all 104 studied organisms have at least one tRNA for Tyr with anticodon GTA (some organisms, for instance H.sapiens, have different tRNAs with the same anticodon, altogether there are 189 different tRNAs with the anticodon GTA in the 104 species), but no organism has a tRNA for His with anticodon ATG. Table 3 lists different codon - reverse codon pairs. It is interesting to note that in the whole superkingdom bacteria the number of tRNA genes for the reverse part of table 3 is always zero, the few exceptions are only in eukaryotes and archaea. Another unique property of all bacteria is that there is no tRNA with anticodons for the self reverse codons UUU (Phe), UCU (Ser), UGU (Cys) and UAU (Tyr). Generally, table 3 shows that anticodons with an A at the third position are strongly preferred, whereas A** anticodons are significantly suppressed.