Evolution of different transcriptional start sites in the human luteinizing hormone and chorionic gonadotropin beta-subunit genes.

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 human 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.