STRUCTURE AND REGULATION OF THE LUTEINIZING-HORMONE RECEPTOR GENE

Studies of the mechanisms controlling the expression of the rat lutein izing hormone receptor gene were pursued by characterization of the ge ne structure and identification of regulatory protein binding domains in the 5'-non-coding region of the gene and of 3' non-coding functiona l domains responsible for generation of the major mRNA forms. The codi ng region of the rat LHR gene contains 10 introns and 11 exons, of whi ch the first 10 exons comprise the hormone binding extracellular domai n and exon 11, the seven transmembrane/G protein coupling module. Seve ral alternative spliced variants of the LHR were identified that confo rm to deletions of complete and/or partial exons. Within the 6.2 kb of the 3'-non-coding region, two functional LBR pA domains (H1) and (H2) produce two sets of major mRNA transcripts, each coding for both holo receptor and the form B splice variant. The H1 pA domain is unique to LHR and may represent a recombinant insertion domain. The functional e fficiency of each pA domain is related to the specific pA signals, dis tal downstream elements, and tissue-specific factors. A TATA-less prom oter region is present within the 173 bp 5' flanking region of the gen e, with Initiator (Inr) elements at transcriptional start sites. Trans cription is dependent on the binding of the Sp1 protein at two Sp1 dom ains that each contribute equally to transcript initiation. Promoter a ctivity is regulated by at least three additional DNA domains, R (-126 6 to -1307 bp), C-box (-42 to -73 bp) and M1 (-24 to -42 bp) that bind multiple trans-factors in a tissue-specific manner. Basal promoter ac tivity is enhanced by a functional M1 domain in LHR-expressing mouse L eydig tumor cells (MLTC) but not in nonexpressing CHO cells. C-box bin ding factors either inhibit promoter activity or block inhibition thro ugh overlapping but not identical DNA binding domains that carry AP-2 and NF-1 elements. Removal of the AP-2 element within the C-box result s in MLTC-specific transcriptional activation that may involve an MTLC M1/C-box interaction. In addition, competition for C-box factors by a n upstream regulatory element (R) that is only inhibitory in CHO cells , indicates that both C-box binding factors compete for this upstream (R) domain in a tissue-specific manner. Competition between the inhibi tory and neutral DNA binding factors within both upstream (R) and prom oter domains (C-box) could provide a mechanism for the control of LH r eceptor gene expression in gonadal cells. These studies have revealed a complex pattern of transcriptional regulation that may reflect targe ts for signal-regulated changes in LH receptor gene expression.