| Abstrakt: |
Luteotropic glycoprotein hormones (LGH) include luteinizing hormone (LH) and chorionic gonadotropin (CG). The order Primates is the only phylogenetic clad known to exhibit more than one LGHbeta subunit gene per haploid genome. In the present study, we report the discovery of a second case of LGHbeta gene replication, in the white (w) rhinoceros (r or rhino). The presence of more than one gene was strongly suggested by a complex banding pattern observed on Southern blots of DNA prepared from two unrelated white rhinos. The existence of two LGHbeta genes per haploid genome was estimated by genomic equivalence assay. However, genomic restriction-site mapping studies, together with other findings, suggested that the replicates are probably not tandemly arranged as occurs in primates. A simple band pattern was observed in Southern blots of four other perissodactyl species, indicating that a single-copy LHbeta gene is the consensus condition. Two distinct white rhino LHbeta genomic clones (wrLHbeta1 and wrLHbeta2) were isolated. The nucleotide sequence of wrLHbeta1 was identical with that of wrLHbeta2, except that the latter lacked the consensus mammalian LGHbeta second intron. Sequences of the TATA-containing proximal 5'-flanking regions of the two genes were homologous to at least -57 relative to the site of pituitary transcriptional initiation. We conclude that wrLHbeta1 is the extant form of the ancestral perissodactyl LHbeta gene, whereas wrLHbeta2 is a randomly integrated cDNA element (processed gene) reverse transcribed from a partially spliced ancestral wrLHbeta1 mRNA. That wrLHbeta2 was heritable demonstrates that wrLHbeta1 was transcribed in gametes or early conceptus cells contributing to the germline at some point in time since the divergence of white rhinos from other members of the family Rhinocerotidae. Furthermore, because homologous proximal (pituitary) promoter sequence is present in wrLHbeta2, it can be concluded that the wrLHbeta1 mRNA template from which wrLHbeta2 is derived was transcribed from a secondary promoter located upstream of the consensus TATA-regulated pituitary promoter. |
