Prolactin gene expression in ovine and caprine mammary gland

The presence of prolactin (PRL) mRNA in the mammary gland of lactating goats and sheep was demonstrated by Northern analysis and RT-PCR. This provides evidence that the PRL gene is transcribed in this tissue. This ectopic expression is not restricted to the lactational period, as PRL transcripts were also found during the last third of pregnancy. Comparison of mammary and pituitary PRL mRNAs showed that they are similar in size but less abundant in mammary gland. In addition, an 847-bp cDNA fragment amplified from mammary retrotranscripts, containing the entire coding region and the major part of the 5' and 3' untranslated regions (UTRs), was found to be identical in sequence to its pituitary counterpart. Primer extension analysis, performed to obtain further information on the structure of the mammary PRL mRNA, has shown that the 5' UTR is 56 nucleotides (nt) long for both species. This is comparable with the size (53 nt) found using the caprine pituitary RNA as template. These results strongly suggest that the PRL gene is not transcribed from a different promoter in mammary gland, as has been demonstrated for placental and lymphocyte cells, but is more likely transcribed from the pituitary-specific promoter. Finally, the presence of PRL mRNA in polysomal fractions suggests that PRL is synthesized in mammary cells.