A cDNA from brain of Xenopus laevis coding for a new precursor of thyrotropin-releasing hormone.

Thyrotropin-releasing hormone (TRH) is found in large amounts in the skin of Xenopus laevis. In this tissue, 3 TRH precursor mRNAs can be detected of which the 2 more expressed encode almost identical proteins. However, Northern blot analysis of TRH precursor mRNAs in the brain of X. laevis revealed the existence of a new mRNA of about 1200 nucleotides which was present along with the larger TRH precursor mRNA identified in the skin. A cloned cDNA of a TRH precursor, corresponding in size to this new mRNA, was isolated and sequenced from a Xenopus brain lambda gt11 library. It encodes a precursor polypeptide which also contains 7 copies of TRH. However, at the amino acid level it differs by about 16% from the corresponding prepro-TRHs from skin. We have also attempted to characterize the gene encoding this prepro-TRH from Xenopus brain. Only the first and part of the second exon could be detected which are separated by an intron containing more than 8000 base pairs. Interestingly, the 5'-flanking region of this gene does not contain the characteristic promoter elements of the mammalian TRH genes suggesting marked differences in the regulation of their expression.