Tissue-specific splicing of pro-enkephalin mRNA

A testis cDNA library derived from 4-6 week-old rats was screened using a 32P-labeled 435 base pair (bp) cDNA probe derived from exon 3 of rat brain proenkephalin (PE). Two positive clones were isolated from an initial screening of approximately 20,000 recombinant bacteriophage plaques. The longest insert (1,500 bp) was sequenced and was found to contain a portion of intron A at the 5'-terminus. Intron B was sliced as it is in the brain, therefore, the domain structure of the cDNA was intron A-exon 2-exon 3, reading in the 5'-3' direction. Since the translational initiator codon is located at the 5'-terminus of exon 2, the reading frame of PE is unaltered, however, within intron A are 4 upstream AUG codons which exist in a favorable context for initiation of translation. It is probable that translation of PE would be inhibited by the presence of these upstream initiator codons, which may explain the discrepancy between the levels of PE mRNA and opioid peptides in the testis. Hypophysectomy prior to the onset of puberty was found to drastically reduce the level of PE mRNA in the testis and epididymus, suggesting that pituitary factors affect the expression of the PE gene in these tissues, either directly or indirectly.