Comparative Biochemistry and Physiology Part B 135 (2003) 449–460 1096-4959/03/$ - see front matter  2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S1096-4959Ž03.00111-8 Cloning and nutritional regulation of a D6-desaturase-like enzyme in the marine teleost gilthead seabream (Sparus aurata)  I. Seiliez*, S. Panserat, G. Corraze, S. Kaushik, P. Bergot Laboratory of Fish Nutrition, Unite mixte INRA-IFREMER, 64310 St-Pee-sur-Nivelle, France ´ ´ Received 2 January 2003; received in revised form 9 April 2003; accepted 9 April 2003 Abstract Marine fish are presumed to have a lower capacity than freshwater fish for the bioconvertion of 18C fatty acids into 20–22C highly unsaturated fatty acids (HUFA). The present work investigated the first step of this pathway, the D6- desaturation, in gilthead seabream. A full-length desaturase-like cDNA was identified from total RNA extracted from viscera of juvenile fish fed for 96 days on an experimental HUFA-free diet containing olive oil as the sole lipid source. The open reading frame encodes a 445-amino acid peptide that contains two membrane-spanning domains, three histidine-rich regions, and a cytochrome b domain, which are characteristic of D6- and D5-desaturases. Predicted protein 5 sequence of seabream desaturase-like indicated a high percentage of identity with mammalian D6-desaturases (approx. 65%). Northern analysis showed two transcripts of approximately 3.7 and 1.8 kb which were highly expressed in fish fed on HUFA-free diet and slightly expressed in fish fed on HUFA-rich diet. The fatty acid profile of the former group was characterized by high levels of D6-desaturation products (18:2 n-9 and 20:2 n-9) with no detectable levels of D5- desaturation product (20:3n-9). These results demonstrate for the first time the presence and nutritional modulation of a D6-desaturase-like cDNA in a marine fish.  2003 Elsevier Science Inc. All rights reserved. Keywords: Cloning; Desaturase; Fish; Gene expression; HUFA; Lipid; Nutrition; Teleost 1. Introduction Fish, like mammals, require a dietary supply of essential fatty acids of the n-3 and n-6 series that they are unable to synthesize (Watanabe, 1982; Sargent et al., 2002). Among teleosts, freshwater species differ from marine species as regards their fatty acid requirements (Sargent et al., 2002). The  Genbank accession number for the gilthead seabream D6- desaturase-like cDNA sequence is AY055749, and Genbank accession number for the gilthead seabream partial D6-desa- turase-like gene sequence is AF525692 *Corresponding author. Tel.: q33-5-59-51-59-51; fax: q 33-5-59-51-54-52. E-mail address: seiliez@st-pee.inra.fr (I. Seiliez). former have a recognized capacity to desaturate— elongate the fatty acids with 18 carbon atoms, linoleic (18:2n-6) and linolenic (18:3n-3) acids, into highly unsaturated fatty acids (HUFA) with 20 and 22 carbon atoms such as arachidonic (AA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids (Henderson and Tocher, 1987; Sar- gent et al., 2002). These HUFA play a major role in eicosanoid production (AA and EPA), vision (DHA), brain development (DHA) and regulation of expression of several genes involved in lipid metabolism (Cook, 1991; Spiegelman and Flier, 1996; Clarke et al., 1997; Forman et al., 1997). In contrast to freshwater fish, marine fish are assumed to have a deficient capacity to bioconvert 18C