A relationship between GC content and coding-sequence length

Since base composition of translational stop codons (TAG, TAA, and TGA) is biased toward a low G+C content, a differential density for these termination signals is expected in random DNA sequences of different base compositions. The expected length of reading frames (DNA segments of sense codons flanked by in-phase stop codons) in random sequences is thus a function of GC content. The analysis of DNA sequences from several genome databases stratified according to GC content reveals that the longest coding sequences—exons in vertebrates and genes in prokaryotes—are GC-rich, while the shortest ones are GC-poor. Exon lengthening in GC-rich vertebrate regions does not result, however, in longer vertebrate proteins, perhaps because of the lower number of exons in the genes located in these regions. The effects on coding-sequence lengths constitute a new evolutionary meaning for compositional variations in DNA GC content. Correspondence to: J. L. Oliver