The border residues of the dihydrofolate reductase domain in Escherichia coli beta-galactosidase correspond to the positions of introns 1 and 5 of dihydrofolate reductase of chicken.

In beta-galactosidase of Escherichia coli residues 820-934 are similar to residues in dihydrofolate reductase of E. coli. Dihydrofolate reductase of E. coli and chicken are also similar and have identical tertiary structures. I used the similarity of the three-dimensional structure of prokaryotic and eukaryotic dihydrofolate reductases to align the chicken dihydrofolate reductase and the similar residues of beta-galactosidase. The positions of introns 1 and 5 of the chicken dihydrofolate reductase gene correspond exactly to the start and the end of the dihydrofolate reductase-like domain in the beta-galactosidase polypeptide chain. This equivalence of intron positions in a eukaryotic gene and domain structure in a prokaryotic protein was interpreted as evidence for a common origin of both genes.