A single gene encodes the β-subunits of equine luteinizing hormone and chorionic gonadotropin

Abstract

Equine (e) CG and LH β-subunits have identical amino acid sequences, including an extended carboxyl-terminal peptide (CTP). This suggests that unlike the corresponding human genes, the β-subunits of eCG and eLH may be encoded by a single gene and share a common proximal promotor region. To explore this, we isolated and characterized the eLH/CGβ gene(s). Data from Southern analyses suggest that the eCGβ and eLHβ subunits are products of the same single copy gene (eLH/CGβ). Overlapping fragments of the eLH/CGβ gene and cDNA were amplified from equine genomic DNA and pituitary gland mRNA by the polymerase chain reaction, cloned, and sequenced. The eLH/CGβ gene spans less than 1.2 kilobasepairs and has three exons that translate a CTP-containing polypeptide identical in sequence to that previously reported for the mature equine protein. There is, however, little amino acid homology shown between the CTP of human or equine CGβ subunit. In addition, unlike the human genes, the same TATAA-like element appears to be involved in directing initiation of transcription of the eLH/CGβ gene in placenta and anterior pituitary. Based upon these differences, we suggest that the CGβ genes evolved independently in humans and equids and that different mechanisms are involved in their patterns of placenta-specific expression.