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We test the hypothesis that introns were used to construct the first genes from small exons, whose protein products represent compact elements of structure. For any three-dimensional structure, a computer program analyzes the structure into a set of modules, segments of the polypeptide chain bounded in space by a maximum diameter, separated by a set of ‘boundary regions’. The ‘boundary regions’ are such that if the gene were divided by an intron in each ‘boundary region’, the protein would be divided into modules less than the specified diameter. Using a set of 32 ancient proteins, which have no introns in prokaryotes, we examine the intron positions in their eukaryotic homologs and show that the introns are correlated with modules of diameter 21, 28 and 33 A, with P values below 0.001.

If genes have been assembled from exon subunits, the frequency with which exons are reused leads to an estimate of the size of the underlying exon universe. An exon database was constructed from available protein sequences, and homologous exons were identified on the basis of amino acid identity; statistically significant matches were determined by Monte Carlo methods. It is estimated that only 1000 to 7000 exons were needed to construct all proteins.

We analyze the three-dimensional structure of proteins by a computer program that finds regions of sequence that contain module boundaries, defining a module as a segment of polypeptide chain bounded in space by a specific given distance. The program defines a set of “linker regions” that have the property that if an intron were to be placed into each linker region, the protein would be dissected into a set of modules all less than the specified diameter. We test a set of 32 proteins, all of ancient origin, and a corresponding set of 570 intron positions, to ask if there is a statistically significant excess of intron positions within the linker regions. For 28-Å modules, a standard size used historically, we find such an excess, with P < 0.003. This correlation is neither due to a compositional or sequence bias in the linker regions nor to a surface bias in intron positions. Furthermore, a subset of 20 introns, which can be putatively identified as old, lies even more explicitly within the linker regions, with P < 0.0003. Thus, there is a strong correlation between intron positions and three-dimensional structural elements of ancient proteins as expected by the introns-early approach. We then study a range of module diameters and show that, as the diameter varies, significant peaks of correlation appear for module diameters centered at 21.7, 27.6, and 32.9 Å. These preferred module diameters roughly correspond to predicted exon sizes of 15, 22, and 30 residues. Thus, there are significant correlations between introns, modules, and a quantized pattern of the lengths of polypeptide chains, which is the prediction of the “Exon Theory of Genes.”

By bending electron microscope grids prior to viewing, it is possible to gain a perspective of the sample which gives a three-dimensional effect. Conventional electron micrographs prepared in this manner produce results which are comparable to those from the scanning electron microscope.