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Abstract

A gene encoding a member of the growing family of regulatory WD-repeat proteins has been identified in Hydra vulgaris. About 80% of its deduced amino acids are identical to RACK1, which has recently been identified in rat as a receptor for activated C kinase. The presence of a consensus sequence believed to be important for interaction with protein kinase C, prompted us to term the new sequence HvRACK1 (Hydra vulgaris RACK1). In situ hybridization revealed an abundant message restricted to 80% of the body column with the strongest signal in the upper body half. Terminally differentiated structures (foot, tentacles and apicalmost hypostomal cells) were completely free of expression. A corresponding prepattern was established in bisected animals at least 10 h before the regenerating structure began to form. In normal animals HvRACK1 transcripts were contained mainly in interstitial cells (i-cells), gland cells and digestive epithelial cells. Depletion of i-cells and their derivatives by hydroxyurea (HU) treatment induced expression of the gene in epithelio-muscular and -digestive cells with the overall expression level remaining stable as confirmed by northern blotting. Polyps consisting almost exclusively of epithelial cells expressed HvRACK1 differentially along the body axis with a maximum in the head (except tentacles and the apicalmost ectoderm of the hypostome) and fading out towards the foot. The enhanced expression level in epithelial cells of HU-treated animals indicates that these might take over regionspecific HvRACK1 functions usually inherent to interstitial cells and certain derivatives of this lineage.