CLONING AND ANALYSIS OF THE CDNA-ENCODING THE HORSE AND DONKEY LUTEINIZING-HORMONE BETA-SUBUNITS

The coding regions of the horse (Equus caballus) and donkey (E. asinus ) luteinizing hormone (LH) beta-subunit transcripts were cloned from p ituitary gland RNA, in order to investigate their relationships to the corresponding equine chorionic gonadotropin (CG) beta-subunits and to further understand the unusual receptor-binding properties of equine LH and CG, The horse and donkey LH beta-subunit sequences were very si milar (97% identity at the nucleotide (nt) level; 93% at the amino aci d (aa) level), confirming their very close evolutionary linkage and al so indicating that the C-terminal extension in both subunits occurred prior to the divergence of horses and donkeys. Furthermore, sequence c omparisons and Southern blot analysis confirmed that in donkeys, as in horses, the pituitary (LH) and placental (CG) beta-subunits are almos t certainly derived from a single gene. This contrasts with primates w hich have a single LH beta gene expressed in the pituitary and a famil y of separate CG beta genes that are expressed in the placenta. The de duced aa sequences also revealed several differences between the horse and donkey LH/CG beta-subunits that could explain their differences i n biological activity. In addition, the study confirmed that the donke y subunit contains an additional Cys residue that is not present in ot her gonadotropin beta-subunits.