The translational start sites of jawless and cartilaginous fish genes.

Nucleotide sequences that surrounded ATG initiation codons were examined in jawless and cartilaginous fish complementary DNA sequences. Both thymidine and cytidine residues were underrepresented at positions near the initiation codon, while an extremely high frequency of purine nucleotides was observed at position -3. Statistical analysis (chi2) indicated that the greatest compositional bias occurred at nucleotide positions -3 and +4, and suggested that a relatively short consensus sequence surrounded AUG initiation codons of primitive fish genes. ATG triplets within 5' leader sequences were flanked by nucleotides different from those that surrounded ATG initiation codons. Dinucleotide frequency analysis indicated a deficiency in TA and an excess in AA around initiation codons. DNA sequence analysis suggested that low CpG conversion occurred 5' to the translation start of primitive fish genes. The conservation of consensus sequences around initiation codons of primitive fish genes underscores the importance of nucleotide composition for initiation of translation.