The cDNA and the gene for pro-opiomelanocortin (POMC) in the zebrafish (Danio rerio) were isolated and analyzed. The gene consists of three exons and two short introns and has a similar overall structural organization as in Homo sapiens. Intron 1 (339 bp) divides the 5' untranslated region from the coding region while intron 2 (1522 bp) is located between the signal peptide and the sequence encoding ACTH. Transcription starts 26 bp downstream of a TATA box and there is one polyadenylation signal in the 3' untranslated region. The cDNA comprises of 964 bp with an open reading frame encoding a 222 amino acid hormone prepropeptide that is split into six putative hormones. Sequence comparison of zebrafish POW to sequences of various other vertebrate species reveals four regions that are highly conserved during the evolution of vertebrates-the N-terminal region, ACTH, beta-NISH, and beta-endorphin, whereas the connecting peptides show a much higher degree of variability. Phylogenetic analysis of the POMC sequences of various vertebrate species resulted in the expected pattern of species evolution. In situ hybridization demonstrated POW expression in a cluster of cells (corticotrophs) in the pituitary of the zebrafish as early as 23 h after fertilization. These findings will facilitate the use of the zebrafish as a model organism in the study of the physiological role of POMC-derived peptides. (C) 2003 Elsevier Science (USA). All rights reserved.