Differential expression of cloned mouse metallothionein sequences in Escherichia coli.

A cDNA for mouse hepatic metallothionein I has been cloned into pBR322 (Mbikay et al., 1981). Although this recombinant plasmid (M135) possesses the metallothionein sequence in the same reading frame as that of beta-lactamase, it fails to direct the synthesis of a fused beta-lactamase-metallothionein protein in Escherichia coli. Another plasmid (M244) was derived from M135 by deleting an internal 390-bp segment made of the 5' noncoding region of metallothionein, the dG-dC tail, and some beta-lactamase sequences. Bacteria harboring the new plasmid now contain in their periplasmic space a cysteine-rich, cadmium-binding protein of 12,000 daltons, as expected. These observations demonstrate that expression of cloned DNA in bacteria could depend as much on its primary structure as on proper insertion in relation to a promoter.