Variants of the 5'-untranslated sequence of human growth hormone receptor mRNA

The human growth hormone receptor (GHR) gene was proposed to contain multiple 5'-noncoding exons (Leung et al., 1987). The exact number and structure of these exons are unknown. As a first step in investigating this point more closely, we decided to clone alternative 5'-noncoding sequences of human liver GHR mRNA. The ligation-mediated single-sided polymerase chain reaction (PCR) was applied for selective amplification of 5'-terminal sequences of human liver GHR cDNA. PCR products were cloned and sequenced. Eight different sequence variants diverging in the 5'-untranslated regions beginning 12 base pairs upstream from the initiating ATG codon were found. One variant seems to represent unspliced or partially spliced GHR mRNA. The remaining variants probably correspond to multiple alternatively spliced forms of GHR mRNA. Homologs for three of these variants were found among previously published 5'-noncoding sequences of GHR cDNA obtained from other species by conventional cDNA cloning. Most of the cloned human liver GHR cDNA variants contain one or more ATG preceding the main GHR open reading frame start of translation. Thus, the GHR genes appeared to be a striking example of a very complex transcription unit.