TYROSINE-HYDROXYLASE IN THE EUROPEAN EEL (ANGUILLA-ANGUILLA) - CDNA CLOTTING, BRAIN DISTRIBUTION, AND PHYLOGENETIC ANALYSIS

We report the isolation of a full-length eel tyrosine hydroxylase (TH) cDNA that is characterized by a long 3' untranslated region and by a diversity restricted to the 3' end owing to the differential use of th ree polyadenylation signals. The longest eel TH mRNA was distinctive i n the presence of four pentameric elements (AUUUA) in the AU-rich 3' n oncoding region. Such a diversity could provide the basis of posttrans criptional or translational regulation of eel TH gene expression. Comp arison of the eel TH sequence with those of other aromatic amino acid hydroxylases (TH, tryptophan hydroxylase, and phenylalanine hydroxylas e) and phylogenetic analysis confirmed that the N-terminal regulatory domain is highly divergent, contrasting with the conservation of the c atalytic core of the enzyme. Molecular phylogenies including the avail able sequences of the three hydroxylase genes suggested that the dupli cation of their common ancestor occurred before the emergence of arthr opods. The regional expression of the eel TH mRNA was studied by semiq uantitative PCR, northern blots, and in situ hybridization and compare d with the immunocytochemical localization of TH protein. The data sho wed that TH mRNA is mostly expressed in the olfactory and hypothalamic areas, whereas sparse TH-expressing cell bodies are present in the te lencephalic region and brainstem. No labeling was detected in the mese ncephalic area, in striking contrast with that found in amphibians and amniotes.