Evolution of Different Transcriptional Start Sites in the Human Luteinizing Hormone and Chorionic Gonadotropin β-Subunit Genes

Abstract

To investigate the mechanisms related to the tissue-specific expression of luteinizing hormone (LH) in the pituitary and chorionic gonadotropin (CG) in the placenta, we compared the transcriptional start sites of the common .alpha.-subunit and structurally related .beta.-subunit genes encoding human LH and CG. The transcriptional start site of the .alpha.-subunit gene expressed in hunabn pituitary and placenta is identical, encoding a mRNA with a 5''-untranslated tract of 100 bases. In contrast, the lengths of the 5''-untranslated tracts for human LH.beta. and CG.beta. mRNA are different. The human LH.beta. gene, like the LH.beta. gene in rat and cattle, transcribes a mRNA with a short 5'' untranslated tract of 9 bases. Although the human CG.beta. gene has a TATAAA box sequence in the same location as the LH.beta. gene, it is not used for initiation of transcription. Instead, the promoter for the CG.beta. gene is located at an upstream site resulting in an extended 5'' untranslated tract for CG.beta. mRNA of 366 bases. These results indicate that recognition of the upstream promoter site in the CG.beta. gene is tissue specific rather than species specific and suggest that the LH.beta. and CG.beta. genes evolved distinct regulatory regions which respond differently to intracellular signals in the pituitary and placenta.