Organization of the human transferrin gene: direct evidence that it originated by gene duplication

We present the characterization of two overlapping human transferrin genomic clones isolated from a liver DNA library. The two clones represent a total length of 24 kilobase pairs and code for 70% of the protein. The organization of this gene region was elucidated by restriction mapping and DNA sequencing. It contains 12 exons, ranging from 33 to 181 base pairs, separated by introns of 0.7-4.9 kilobase pairs. This gene can be divided into two unequal parts corresponding to the known domains of the protein. Each part is essentially composed of an equal number of exons; introns interrupt the coding sequences, creating homologous exons of similar size in each moiety. Moreover, the pattern of intron interruption of the codon sequence is identical for all the analyzed homologous exon pairs. Comparison with the organization of the ovotransferrin gene shows an identical exon size distribution. These data confirm, at the gene level, the hypothesis that transferrins originated by a gene-duplication event. A model accounting for the origin of the human transferrin gene is presented.