Nucleotide sequence of transferrin cDNAs and tissue-specific expression of the transferrin gene in Atlantic cod (Gadus morhua)

Atlantic cod (Gadus morhua) transferrin cDNAs were isolated from a liver cDNA library using a cod transferrin-derived polymerase chain reaction product as a hybridization probe. The composite nucleotide sequence of two overlapping clones was 2223 bp in length excluding the poly(A) sequence and was equivalent to 87% of the 3' end of the Atlantic salmon transferrin cDNA sequence. Comparison of the deduced amino acid sequence of cod, salmon, Xenopus and several mammalian transferrins revealed that the two fish sequences are more similar with respect to their amino acid sequence and the position of additions/deletions than to other vertebrate transferrins. Conservation of the iron-binding domains and cysteine residues involved in disulphide bridges indicates that all transferrins share similar tertiary structure and support the hypothesis that extant vertebrate transferrin genes were derived from a gene duplication before the divergence of fish, frogs and mammals. Cod transferrin mRNA was detected in both brain and liver RNA and to a much lesser extent in RNA isolated from kidney and heart in contrast to salmon and several other vertebrates in which the transferrin gene is not expressed in brain.