Identification of the transcriptionally active genes of the chorionic gonadotropin beta gene cluster in vivo.

The chorionic gonadotropin beta (CG beta) subunit is encoded by a multigene cluster composed of six homologous sequences (genes or pseudogenes). They are primarily distinguished by sequences in the 5' nontranslated region of the first exon. To determine which CG beta genes are active in vivo, we employed the reverse transcription-polymerase chain reaction technique. DNA complementary to RNA from placenta tissue and a choriocarcinoma cell line was subjected to polymerase chain reaction with CG beta-specific primers. The amplified DNA was cloned into M13 and sequenced. Most of steady-state CG beta mRNAs are transcribed from CG beta genes, 5, 3, and 8. The level of expression is beta 5 greater than beta 3 = beta 8 greater than beta 7, beta 1/2. Transcripts from the CG beta 1 and CG beta 2 genes, which were previously considered pseudogenes because of their noncanonical splice site, were detected. These CG beta transcripts arising from alternative splicing sites in the CG beta 1/2 genes were also detected in polysomes which suggests they are translation-competent. Northern blotting with a CG beta 1/2 probe revealed low amounts of CG beta 1/2 transcripts which were smaller than the transcripts of the other genes. The results indicate that at least five, and possibly all six, CG beta genes are transcribed in vivo, and at least one of them (CG beta 1 and/or CG beta 2) undergoes alternative splicing.