Hydra regenerate throughout their life. We previously described early modulations in cAMP-response element-binding protein (CREB) DNA-binding activity during regeneration. We now show that the Ser-67 residue located in the P-box is a target for posttranslational regulation. The antihydra CREB antiserum detected CREB-positive nuclei distributed in endoderm and ectoderm, whereas the phosphoSer133-CREB antibody detected phospho-CREB-positive nuclei exclusively in endodermal cells. During early regeneration, we observed a dramatic increase in the number of phospho-CREB-positive nuclei in head-regenerating tips, exceeding 80% of the endodermal cells. We identified among CREBbinding kinases the p80 kinase, which showed an enhanced activity and a hyperphosphorylated status during head but not foot regeneration. According to biochemical and immunological evidence, this p80 kinase belongs to the Ribosomal protein S6 kinase family. Exposure to the U0126 mitogen-activated protein kinase kinase inhibitor inhibited head but not foot regeneration, abolished CREB phosphorylation and activation of the early gene HyBra1 in head-regenerating tips. These data support a role for the mitogen-activated protein kinase͞ribosomal protein S6 kinase͞ CREB pathway in hydra head organizer activity. S pecies that display regenerative capacities can be found in most animal phyla, from invertebrates to vertebrates, likely representing an ancestral feature lost several times during evolution. The central issue concerning regeneration is the unfolding of developmental programs in adult tissues, ending ultimately in a de novo morphogenesis. That issue is poorly understood, but some stages might rely on conserved ancestral mechanisms. Hydra, which belong to the Cnidaria phylum, a sister group to bilaterians, bud and regenerate throughout their life. Hence their developmental programs stay permanently accessible. Hydra are made up of two cell layers, ectoderm and endoderm, separated by the mesoglea. After midgastric section, regeneration of the missing part is achieved within 3 days, resulting from differentiation of cells from the gastric region in the absence of cell proliferation during the first day.Although early modulations in gene expression have been identified during regeneration (see ref. 1), little is known about the signaling mechanisms that control regeneration (2, 3). Diacylglycerol treatment induces formation of multiple heads (4), whereas inhibition of PKC and mitogen-activated protein kinase (MAPK) pathways selectively block head regeneration and initiation of budding (5-7). In hydra, conserved regulatory elements have been tested in gel retardation assays, and a significant modulation during both apical and basal regeneration was detected with the cAMP response element (CRE). The hydra CRE-binding protein (CREB) gene was cloned, showing a high level of similarity with vertebrate cognate genes (8, 9). In vertebrates, CREB mediates the response to a large array of extracellular signals to the nucleus through posttranslational modifi...