Messenger RNAs encoding the beta subunits of guinea pig (Cavia porcellus) luteinizing hormone (gpLH) and putative chorionic gonadotropin (gpCG) are transcribed from a single-copy gpLH/CGbeta gene

Abstract

Neither gene locus nor gene sequence characterizations have been reported for the beta subunits of guinea pig (gp) LH and putative gp chorionic gonadotropin (CG). Descriptions of this locus would allow comparison with functionally relevant molecular genetic features of other species' homologous loci including the single-copy equid LH/CGbeta gene and the primate LHbeta-CGbeta gene cluster locus. Contiguous cDNA and genomic DNA fragments spanning the entire mature coding sequence of gpLHbeta mRNA, gpCGbeta mRNA and a homologous gpLH/CGbeta gene were amplified using PCR methodologies. With the exception of one silent mutation, the two cDNA and the genomic sequences were identical where they overlapped. Comparison of guinea pig coding sequence with LHbeta, CGbeta and LH/CGbeta sequences of other vertebrate species revealed the following order of similarity expressed as per cent coding sequence identity: rhinoceros LHbeta (83.6%)>pig LHbeta (81.8%)>donkey LH/CGbeta=bovine LHbeta (81.5%)> horse LH/CGbeta (80.6%)>dog LHbeta (79.7%)>human LHbeta (78.2%)>rat LHbeta (77.9%)>human CGbeta (75.8%)>turkey LHbeta (52.7%); values that are generally consistent with recently postulated phylogenetic relationships. Like the consensus mammalian LHbeta gene, the 5'-flanking region of the gpLH/CGbeta gene contains a single TATA sequence 37 bp upstream of the translation start codon. The first in-frame stop codon occurred at codon position +122 which is consistent with the 121 amino acid residue length of the consensus mammalian mature LHbeta peptide. To estimate gene copy number, full-length gpLHbeta cDNA was radiolabeled and hybridized to Southern blots of guinea pig genomic DNA digested with a panel of six restriction endonucleases. The resulting simple hybridization pattern strongly suggested that there is a single-copy gpLH/CGbeta gene. Northern analysis of total pituitary RNA using the same probe indicated that gpLHbeta transcript size is indistinguishable from that of consensus mammalian pituitary LHbeta mRNAs ( approximately 750 nucleotides). Despite amplifying gpCGbeta from placental RNA, positive signal was not detected in Northern blot lanes containing guinea pig total RNA prepared from placentae collected at three gestational ages (17.3 days, 24.3 days and 68 days (term)). Other data suggest that inability to detect Northern blot signal could have been due to low relative tissue concentrations of gpCGbeta transcript and/or sampling at gestational time-points that missed peak periods of mRNA expression. We conclude that, with respect to gene copy number, coding sequence and pituitary mRNA size, the gpLH/CGbeta gene locus reflects the CTP-less consensus mammalian LHbeta condition. However, based on the capacity of this single-copy gene to express in both pituitary and placental tissues, gpLH/CGbeta also exhibits functional similarities with the single-copy equine LH/CGbeta locus.