Rat Pit-1 Stimulates Transcription in Vitro by Influencing Preinitiation Complex Assembly

The anterior pituitary-specific transcription factor, Pit-1, activates prolactin, growth hormone, TSHβ, growth hormone receptor genes and autoregulates the pit-1 gene. Its mechanism of transcription activation is unknown. Using immobilized DNA templates and order-of-addition transcription assays, it is shown that Pit-1 is required during pre-initiation complex assembly to activate the prolactin gene in vitro. Using prolactin promoters containing point mutations in the distal TATA box, it is also demonstrated that Pit-1 activation in vitro is not mediated simply by repressing the upstream, alternative promoter. Experiments show that a preformed class II pre-initiation complex is refractory to Pit-1 influence. The data indicate that Pit-1, and perhaps other members of the POU-protein family, activate transcription by influencing the type pre-initiation complex assembled on target promoters.