1097-2765712001JanMol CellTwo-step regulation of left-right asymmetric expression of Pitx2: initiation by nodal signaling and maintenance by Nkx2.137149137-49Pitx2 is left--right (L--R) asymmetrically expressed initially in the lateral plate and later in primordial visceral organs. The transcriptional regulatory mechanisms that underlie L--R asymmetric expression of Pitx2 were investigated. Mouse Pitx2 has a left side-specific enhancer (ASE) that mediates both the initiation and maintenance of L--R asymmetric expression. This element contains three binding sites for the transcription factor FAST. The FAST binding sites function as Nodal-responsive elements and are sufficient for the initiation but not for the maintenance of asymmetric expression. The maintenance requires an Nkx2-5 binding site also present within the ASE. These results suggest that the left-sided expression of Pitx2 is directly initiated by Nodal signaling and is subsequently maintained by Nkx2. Such two-step control may represent a general mechanism for gene regulation during development.ShiratoriHHDivision of Molecular Biology, Institute for Molecular and Cellular Biology, Osaka University, 1-3 Yamada-oka, Suita, Osaka 565-0871, Japan.SakumaRRWatanabeMMHashiguchiHHMochidaKKSakaiYYNishinoJJSaijohYYWhitmanMMHamadaHHengJournal ArticleResearch Support, Non-U.S. Gov't