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A cDNA clone for the pre-alpha subunit of the pituitary glycoprotein hormones has been isolated from a bovine pituitary cDNA library through the use of a pool of synthetic oligodeoxynucleotide probes. This clone, designated pB alpha, contains a 564-base pair insert which includes a portion of the signal sequence, the entire coding sequence of the mature protein, and 224 base pairs of the 3'-untranslated sequence. As expected, the nucleotide and amino acid sequence of the mature bovine alpha subunit was homologous to the sequences reported for humans and rodents, with the most extensive homology occurring between bovine and rodents (85-90%). However, a comparison of the 3'-untranslated regions of pre-alpha subunit mRNA from three different mammalian species indicated that in bovine and rat, or in human and rat, these sequences have rapidly diverged, yielding respective homologies of 21 and 36%. In contrast, the sequence homology observed between the 3'-untranslated regions of bovine and human was 79%, which approaches the level of homology shared by their coding sequences. Thus, the conservation of the 3'-untranslated sequence in bovine and human pre-alpha subunit mRNA may be an indication that this region is functionally significant in these two species.

Cell-free translation was used to initially characterize the major mRNA species present in the bovine anterior pituitary as a function of development. The only detectable change in translation products, which occurred during the transition from fetus to adult, was a reversal in the relative ratio of pituitary growth hormone and prolactin. Subsequent hybridization analysis with cloned growth hormone and prolactin cDNA probes indicated that growth hormone mRNA comprised over 40% of the total fetal mRNA and was 50- to 100-fold higher than prolactin mRNA. The steady state levels of growth hormone mRNA remained relatively constant throughout gestation. In contrast, prolactin mRNA levels, which were low early in gestation, increased during development to become the principal mRNA in the adult pituitary. Since growth hormone and prolactin are synthesized and secreted by specialized cells (somatotrophs and mammotrophs, respectively) immunochemical staining was used to determine whether the changes in the mRNA levels for these two hormones were a reflection of specific cell proliferation. For growth hormone, there was a close correlation between the number of somatotrophs and the relative levels of growth hormone mRNA. In contrast, the increase in prolactin mRNA exceeded the increase in the number of mammotrophs. Thus, the cellular concentration of growth hormone mRNA remains relatively constant during development, while the cellular concentration of prolactin mRNA increases by more than an order of magnitude.