Accumulation of untranslated metallothionein mRNA in antarctic hemoglobinless fish (icefish)

Icefish (Notothenioidei, Channichthyidae) are a group of Antarctic fish displaying unique phenotype characteristics. Besides the reported lack of hemoglobin, icefish have a very low metallothionein (MT) content compared to Antarctic red-blooded fish. Despite the low amount of MT protein in icefish, cDNA encoding two MT isoforms could be produced from total hepatic RNA by RT-PCR. The mRNA 5' end fragments were amplified by 5'-RACE. Steady-state mRNA levels were assessed in both icefish and red-blooded fish by high stringency hybridization of the MT probe with total RNA. The results showed the presence of large amounts of MT mRNA in icefish liver. A further increase in MT mRNA was observed in Cd-treated icefish which was accompanied by the appearance of large amounts of MT protein. A comparison of the sequences of the 5' ends of the mRNAs showed the lack of a short motif in the 5'-UTR of only one of the two MT isoforms. On the basis of these results, we suggest that translation of the mRNA encoding an MT isoform is regulated in icefish by specific motifs in the 5' end.