Ptx1 regulates SF-1 activity by an interaction that mimics the role of theligand-binding domain

Ptx1 (Pitx1) is a bicoid-related homeobox transcription factor expressed fr om the onset of pituitary development. It was shown to cooperate with cell- restricted factors, such as Pit1, NeuroD1/PanI and steroidogenic factor 1 ( SF-1), to establish a combinatorial code conferring lineage- and promoter-s pecific gene transcription in the pituitary. Transcriptional synergism betw een Ptx1 and SF-1 on two SF-1 target genes, pituitary luteinizing hormone b eta and Mullerian-inhibiting substance (MIS), requires SF-1 binding to DNA and appears to result from direct physical interaction between these two pr oteins. The interaction between the C-terminus of Ptx1 and the N-terminal h alf of SF-1 results in transcriptional enhancement that equals the activity of a constitutively active SF-1 mutant and that may mimic the effect of a still unidentified SF-1 ligand. Thus, the unmasking of SF-1 activity by Ptx 1 mag represent a developmental mechanism to alleviate the need for SF-1 li gand in transcription and, possibly, at critical times during organogenesis .