To investigate the factors involved in the control of metallothionein
(MT) expression in fish, genomic clones encoding the MTs from the Cd-t
olerant fish (pike and stone leach) were isolated. Sequencing revealed
putative metal regulatory elements (MREs) in the 5'-regions of both M
T genes. Comparison of their composition and location showed little di
stinction between different fish species. However, although high overa
ll homology exists between fish MTs and their mammalian counterparts,
the 5'-regions of the fish genes are distinct, not only in the organis
ation of their MREs but also in the absence of other transcription fac
tor binding sites.