The Pit-1 beta domain dictates active repression and alteration of histoneacetylation of the proximal prolactin promoter

A critical problem in current molecular biology is to gain a detailed under standing of the molecular mechanisms by which related transcription factor isoforms with identical DNA sequence specificity mediate distinct transcrip tion responses. Pit-1 and Plt-1 beta constitute such a pair of transcriptio n factor isoforms. Pit-1 enhances the Ras signaling pathway to the prolacti n promoter, and Pit-1 beta represses basal prolactin promoter activity as w ell as Ras signaling to the prolactin promoter in pituitary cells. We have previously demonstrated that the beta-domain amino acid sequence dictates t he transcriptional properties of Pit-1 beta. Here, we show that five hydrop hobic beta-domain residues are required for Pit-1 isoform-specific repressi on of Res signaling, and we demonstrate that sodium butyrate and trichostat in A, pharmacological inhibitors of histone deacetylation, as well as viral Ski protein, a dominant-negative inhibitor of recruitment of N-CoR/mSin(3) histone deacetylase complexes, specifically reverse beta isoform-specific repression of Ras signaling. Moreover, we directly demonstrate, with a chro matin immunoprecipitation assay, that the Pit-1 beta isoform alters the his tone acetylation state of the proximal prolactin promoter. This differentia l analysis of Pit-1/Pit-1 beta isoform function provides significant insigh ts into the structural determinants that govern how different transcription factors with identical DNA sequence specificity can display opposite effec ts on target gene activity.