GROWTH-HORMONE (GH) RECEPTOR AND GH-BINDING PROTEIN MESSENGER RIBONUCLEIC-ACIDS WITH ALTERNATIVE 5'-UNTRANSLATED REGIONS ARE DIFFERENTIALLYEXPRESSED IN MOUSE-LIVER AND PLACENTA

Two 5'-untranslated regions (5'UTRs) with distinctly different sequenc es, designated 5'UTR L1 and 5'UTR L2, were obtained by amplification o f complementary DNA from mouse liver and placenta with primers complem entary to sequences from the hormone-binding domain common to GH recep tor (GHR) and GH-binding protein (GHBP) messenger RNAs (mRNAs). The pr esence of an open reading frame in the 5'UTR L2 and the high GC conten t of this sequence suggest that mRNAs containing this 5'UTR may be tra nslated with a lower efficiency than those containing 5'UTR L1. Expres sion studies showed that 5'UTR L1 and 5'UTR L2 are present in GHR and GHBP mRNAs in both tissues. However, the relative expression of the tw o 5'UTRs differs between liver and placenta and in liver from differen t physiological states. The different expression patterns of the L1 an d L2 5'UTRs predict that the corresponding 5'-noncoding exons of the G HR/GHBP gene are associated with different regulatory elements. The ex pression patterns of the 5'UTRs also indicate that there is a linkage between the 5'UTR present in GHR/GHBP gene transcripts and the alterna tive splicing of these transcripts to yield either GHR or GHBP mRNAs. The 5'-noncoding exon used for transcription of the GHR/GHBP gene, the refore, may be involved in regulating both the ratio of GHR to GHBP tr anscripts and the efficiency of translation of these transcripts. Tran scription from the different 5'-noncoding exons of the GHR/GHBP gene t hus may be a critical element in the regulation of the expression of G HR and GHBP and thereby in the control of the responses of different t issues to GH.