Icefish (family Channichthyidae, suborder Nothothenioidei) are a group of Antarctic fish that have evolved unique phenotypes in order to adapt to the environment in which they live. Besides the lack of haemoglobin and the drastic reduction in the number of erythrocyte-like cells, another striking feature of the icefish is that their liver is devoid of metallothionein. These cysteine-rich heavy-metal-binding proteins are usually present in large amounts in a large variety of organisms, from bacteria to mammals. Despite the failure to detect appreciable levels of metallothionein in icefish liver, a cDNA encoding metallothionein was produced from total RNA by reverse transcriptase PCR. The icefish metallothionein showed high percentage identity with metallothionein from Trematomus bernacchii, a red-blooded Antarctic fish in which a normal content of hepatic metallothionein was found. Steady-state mRNA levels were assessed in fish liver by high-stringency hybridization of the metallothionein probe with total RNA. The results showed that icefish livers retain large amounts of untranslated metallothionein mRNA. The stability of the icefish transcript might be correlated with the lack of specific motifs in the untranslated 3ƀ ends of mRNA.

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