Structural organization of the 3′ half of the rat thyroglobulin gene

We report the structural organization of an 80 Kb segment of rat DNA,which encodes for about 40% of Thyroglobulin mRNA at the 3′ end. The codogenic information included in this segment is splitted in 17 exons of homogeneous size (about 200 bp). The seven exons at the extreme 3′ end have been precisely defined by DNA sequence analysis. No clear sequence homology is found among the exons, even though their coding capacity is quite similar, from 55 to 63 aminoacids residues. We located 2 hormonogenic (T₄ forming) sites on the extreme 3′ end of the gene in different exons. The DNA sequence coding for these functional sites shows a 70% homology in a 50 nucleotides segment. In addition we found a remnant of this sequence in other exona of the gene. Two large introns have been found on the 3′ end of the gene: one is 17 Kb and the other one is more than 30 Kb long. On the basis of these findings and of preliminary studies on the remaining 5′ end of the gene, we can predict that the minimum length of the rat TGB gene will be 150 Kb, which makes this gene the largest so far identified eukaryotic gene. We propose in addition that the 3′ end exona arose by duplication of a common ancestor.