MOLECULAR-CLONING OF THE RHESUS GLYCOPROTEIN HORMONE ALPHA-SUBUNIT GENE

A rhesus monkey genomic library was screened with a cDNA for the glycoprotein hormone alpha-subunit. Genomic clones hybridizing with exon-specific probes were selected and the DNA sequences were determined for 1.6 kb of 5'-flanking DNA, all four exons, the second and third introns, all exon-intron junctions, and 357 bp of 3'-flanking DNA. Comparison with the 236 bp of 5'-flanking sequence data available for the human alpha-gene indicates an overall homology of 95%. Primer extension analysis of rhesus placental and pituitary mRNA demonstrated that transcription initiation is identical to that in the human placenta. The rhesus gene contains an element nearly identical (21/22 bases) to the placental tissue-specific element described for the human-alpha gene. The rhesus gene has only one copy of the cAMP-response element (CRE), which is present as a direct repeat in the human gene. The rhesus CRE contains the consensus core sequence TGACGTCA with the cytosine in the fourth position that is essential for placental expression of the human gene. The 5'-flanking region also has elements highly homologous to the consensus estrogen and progesterone/glucocorticoid response elements, as well as thyrotrope-specific and Pit-1-like binding sites described in rodent genes. The nucleotide sequence of four exons (predicted mRNA) have an aggregate homology of 92.7% with the human sequence. However, a 12-bp insertion to the second exon results in the addition of 4 amino acids to the amino-terminal end of the protein; these are homologous with the proteins of nonprimates but are lacking in the human alpha-subunit. The amino acid sequence of the deduced protein was slightly more homologous with the bovine than the human protein (91.60% vs. 89.60%). Thus, the rhesus glycoprotein alpha-subunit gene codes for a protein whose structure somewhat more closely resembles that of lower species, but the 5'-flanking DNA of the gene has gained the elements necessary for transcription in the placental syncytiotrophoblast which distinguishes the primate placenta from the other species examined.