The role of the genetic code in generating new coding sequences inside existing genes

The genetic code has a very interesting property—it generates an open reading frame (ORF) inside a coding sequence, in a specific phase of the antisense strand with much higher probability than in the random DNA sequences. Furthermore, these antisense ORFs (A-ORFs) possess the same features as real genes—the asymmetry in the nucleotide composition at the first and second positions in codons. About two thirds of the 2997 overlapping ORFs in the yeast genome possess this feature. Thus, the question arises: has this feature of the genetic code been exploited in the evolution of genes? We have searched the FASTA data bases for homologies with the antisense translation products of a specific class of genes and we have found some sequences with relatively high homology. Many of them have scores which could be randomly found in the searched data bases with a probability lower than 10−6. We conclude that some genes could arise by positioning a copy of the original gene under a promoter in the opposite direction in such a way that both, the original gene and its copy initially use the same nucleotides in the third, degenerated positions in codons.