Many Hox proteins are thought to require Pbx and Meis co-factors to specify cell identity during embryogenesis. Here we demonstrate that Meis3 synergizes with Pbx4 and Hoxb1b in promoting hindbrain fates in the zebrafish. We find that Hoxb1b and Pbx4 act together to induce ectopic hoxb1a expression in rhombomere 2 of the hindbrain. In contrast, Hoxb1b and Pbx4 acting together with Meis3 induce hoxb1a, hoxb2, krox20 and valentino expression rostrally and cause extensive transformation of forebrain and midbrain fates to hindbrain fates, including differentiation of excess rhombomere 4-specific Mauthner neurons. This synergistic effect requires that Hoxb1b and Meis3 have intact Pbx-interaction domains, suggesting that their in vivo activity is dependent on binding to Pbx4. In the case of Meis3, binding to Pbx4 is also required for nuclear access. Our results are consistent with Hoxb1b and Meis3 interacting with Pbx4 to form complexes that regulate hindbrain development during zebrafish embryogenesis.

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