The turkey transcription factor Pit-1/GHF-1 can activate the turkey prolactin and growth hormone gene promoters in vitro but is not detectable in lactotrophs in vivo.
Although alternative splicing has been shown to give rise to isoforms of a number of transcription factors, such isoforms have not previously been detected for the POU homeodomain protein Pit-1. Screening of a rat pituitary GH3 cell cDNA expression library yielded a clone, termed pCMVPit-1a, encoding a 35.8 kD protein (Pit-1a) containing a 26 amino acid insert in the Pit-1 trans-activation domain. The position of the insert, plus Southern blot analysis, implied that Pit-1a mRNA arises by alternative splicing of the Pit-1 gene transcript. Pit-1a mRNA was detected in GH3 rat pituitary tumor cells at levels about 1/7 that of Pit-1 mRNA. Pit-1a mRNA-specific sequences were also detected in rat and mouse pituitary, and in mouse thyrotropic tumor TtT cells. DNA mobility shift assays showed that Pit-1a binds specifically to Pit-1 binding sites in the proximal prolactin promoter, but produces DNA-protein complexes of markedly different mobilities than Pit-1. In stably transfected CHO cells which accumulated approximately equal levels of either of the two proteins, Pit-1 trans-activated a prolactin promoter-driven CAT construct, while Pit-1a yielded no detectable transactivation, implying a trans-activation ratio for Pit-1a/Pit-1 of less than 0.05. Thus, the insertion of 26 amino acids of similar composition into the activation domain of Pit-1 has at once affected both the mode of binding of this protein and its ability to function as a trans-activator.