Mutation, selection and the amino acid hydropathic character: a study on receptor genes involved in immune and non-immune functions

In some mRNA sequences, namely those of formyl peptide receptors and chemokine CXC receptors 4, it has been observed that the second nucleotide (nt) of the coding triplets is significantly more highly conserved than the first nt and the correlation between the conservation indexes of the first two nt is positive and significantly higher than the "basic" correlation usually found between adjacent nt. A theoretical analysis demonstrated that random mutations in the first nt preserve hydrophobicity in 73 % of triplets coding for hydrophobic amino acids (aa) and hydrophilicity in 77 % of triplets coding for hydrophilic aa, while random mutations in the second nt preserve hydrophobicity in 18 % of triplets coding for hydrophobic aa and hydrophilicity in 53 % of triplets coding for hydrophilic aa. When the triplets which had changed their hydropathic aa coding character underwent a second random mutation in the previously unmutated first or second nt, an additional 11 % of the originally hydrophobic-coding triplets reverted to hydrophobicity and an additional 14 % of the originally hydrophilic-coding triplets reverted to hydrophilicity. This analysis provides a rationale for why a higher number of mutations in the second nt are presumably negatively selected and a number of double mutations in the first and second nt presumably are positively selected, in cases when a mutation in one of the two is not reverted.