The cDNA for the beta-subunit of human chorionic gonadotropin suggests evolution of a gene by readthrough into the 3'-untranslated region.

A 579-base pair approximately full-length cDNA which codes for the 145-amino acid long beta-subunit of human chorionic gonadotropin (HCG) has been cloned in the plasmid vector pBR322 and its complete nucleotide sequence determined. A hydrophobic presequence of 20 amino acids can be identified from the nucleotide sequence. The amino acid sequence of the beta-subunit is known to be related to those of the beta-subunits of the other glycoprotein hormones LH, FSH and TSH, but the beta-subunit of HCG is unique in that it contains a C-terminal extension of about 30 amino acids which has no homologous counterpart in the other three hormones. Analysis of the beta HCG cDNA nucleotide sequence suggests that this extension may have arisen by the loss of the termination codon of an ancestral beta-like gene so that most of what was previously the 3'-untranslated region now codes for protein. The beta-subunit of HCG terminates with the codon UAA located 16 bases before the poly(A) in the sequence AAUAAA. This sequence is believed to be a recognition signal involved in either polyadenylation or processing and therefore has a dual role in this gene, serving both a coding and a regulatory function.