Table 1 Correlation of codon strength and amino acid properties.

Averaged values (per column, in our scheme) of quantified dinucleoside monophosphate properties (codon and anticodon values give the same average, because of the codon-anticodon symmetry) and amino acid properties for strong, mixed and weak codons. Each row represents one of the measures published by Jungck (1978; This paper contains (in its Table 1) all detailed references as well as a short note to the determination procedure.).

Jungck (1978) collected 15 different measures of amino acid properties, as well as three measures for dinucleoside monophosphates. For all of these 18 measures we arranged a table with 8 rows and 4 columns corresponding to the scheme in Fig.2 (for AU(G/A) we took the Met values (e.g. vertebrate mitochondrial code), for UA(G/A) the Tyr values (mitochondrial flatworm code)). Then we analyzed all row and column sums. The row sums show a strong monotonicity just for the three dinucleoside monophosphate measures and for the hydropohobicity measure of Levitt. However, amazingly, the column sums of nearly all measures are perfectly correlated to the corresponding codon-anticodon binding strength in the sense of Lagerkvist (1978, 1981), in the following simply denoted as codon strength. This is demonstrated in Table 1. For this table we averaged the column sums of the second and third column, giving one “mixed codons” column. As can be seen in Table 1 there are just two exceptions. In the polarity measure of Zimmerman, the deviation is only very weak and in contradiction to all other measures, here the values for the amino acids vary by orders of magnitude. A problem only arises for the three hydrophobicity measures: The two monotonic measures “Levitt” and “BullBreese” are anticorrelated, and the “Jones” measure is not monotonic. The anticorrelation was already found by Jungck (1978), but he did not comment on this.

The fact that the order of the second and third column is not fixed is also underlined by an individual consideration of the two mixed codon columns, instead of the averaging done in Table 1. In about half of the cases the order of the second and third column should be exchanged to guarantee the strong monotonicity of the amino acid measures as function of the column number.

The observed pattern of strong correlation between amino acid properties and codon strength (considers just the first two nucleotides) implies that both first positions together, and not the first or second position alone must have been important for the amino acid – codon assignment in the evolution of the genetic code.