Regulation of metallothionein--thymidine kinase fusion plasmids injected into mouse eggs.

A plasmid was constructed with the promoter/regulatory region of the mouse metallothionein-I gene fused to the structural gene of herpesvirus thymidine kinase. When mouse eggs were microinjected with this plasmid and incubated with cadmium (a natural inducer of metallothionein gene transcription) thymidine kinase activity increased approximately 10-fold compared with control eggs not exposed to cadmium. Analysis of a set of deletion mutants revealed that the minimum sequence required for cadmium regulation lies within 90 nucleotides of the transcription start site.