Project Summary Embryonic patterning requires intercellular signaling pathways that trigger molecular switches to precisely control cell fates. One fundamental step in development is patterning the body axes. Animal anterior-posterior patterning requires the Wnt pathway, which acts through the transcription factor (TF) TCF and its coactivator ?- catenin. Much is known about how classic targets of this pathway are transcriptionally activated in response to Wnt. Wnt induces nuclear accumulation of ?-catenin, which complexes with TCF and converts it from a transcriptional repressor to an activator. Many targets, however, are instead repressed by Wnt, and the mechanisms for this repression are less well understood. Wnt can induce expression of transcriptional repressors, leading to indirect target repression, but emerging evidence from mouse, Drosophila, and C. elegans has identified direct ?opposite? targets, activated by TCF alone and repressed by TCF:?-catenin in response to signaling. The C. elegans embryo is an ideal system to define regulatory mechanisms of these opposite Wnt targets. Almost all cell divisions are patterned by Wnt; after each cell division the posterior sister cell has high Wnt pathway activity, and the anterior sister cell has the Wnt pathway inactive. Our laboratory has identified dozens of genes expressed specifically in either anterior or posterior sister cell-derived lineages and showed that the posterior sister-specific genes are classic Wnt targets, activated when Wnt is active and repressed when Wnt is inactive. My preliminary data indicate that many genes expressed in anterior sister cells are regulated by the Wnt pathway and I propose to test the hypothesis that these genes are direct opposite Wnt targets. I will first test if POP-1/TCF, SYS-1/ ?-catenin, and Wnt ligands and receptors are necessary for proper activation or repression of each of five anterior-specific genes and identify the enhancers controlling their expression. I will identify sites bound by POP-1 throughout the genome, and test whether any are necessary for proper expression of the same five opposite Wnt targets as well as genome-wide. For targets without POP-1 binding, suggesting indirect regulation by Wnt-regulated repressors, I will combine detailed cis- regulatory analysis of their enhancers with our catalog of lineage-specific TF and classic Wnt target expression to identify anterior genes indirectly regulated by Wnt through transcriptional repressors. This work will test whether direct regulation by TCF is a common mechanism for regulation of opposite Wnt targets in C. elegans, will begin to identify mechanisms for this regulation, and will inform future work in this and other systems.