Structural organization of the 5′ region of the human thyroglobulin gene

Abstract

Sequence analyses of bovine and human thyroglobulin (Tg) cDNA have demonstrated that the 5′ region of the mRNA encodes a domain responsible for thyroid hormone synthesis and exhibits striking internal repetition. Knowledge of the organization of the corresponding chromosomal DNA region would provide insight as to how such a structure has evolved. A human genomic DNA library was screened by hybridization in situ, using a bovine Tg cDNA probe corresponding to 2.8 × 10³ base pairs at the 5′ end of the mRNA. Out of 3 × 10⁵ phage plaques, four were scored as positive and yielded three different phages containing thyroglobulin sequences. Selected human Tg cDNA probes were used to order the phages and to identify overlapping regions. Electron microscopy of hybrids between human Tg mRNA and the phage DNA was performed to determine the intron/exon organization of this region. The following conclusions were reached. (a) About 4 × 10⁴ base pairs corresponding to the 5′ region of the gene have been isolated as three overlapping recombinant phages. (b) The three phages cover altogether 2.9 × 10³ base pairs of exonic sequence at the 5′ end of the mRNA. (c) Out of the 11 exons identified in this region, 9 were of a size similar to that of the 3′ exons characterized previously (≤ 200 base pairs); exons 9 (1.12 × 10³ base pairs) and 10 (0.56 × 10³ base pairs) were exceptions to this rule. (d) The phage nearest the 5′ end contains about 9 × 10³ base pairs of sequence located upstream from the gene. The avalability of clones covering the region upstream from the thyroglobin gene will provide the basis for the identification of sequences involved in its transcriptional control by thyroid‐stimulating hormone (thyrotropin).