Expression and regulation of a human metallothionein gene carried on an autonomously replicating shuttle vector

A human metallothionein (MT) gene was inserted into a bovine papillomavirus (BPV) vector. The chimeric vector (pMTII-BPV) transforms rodent fibroblasts to a cadmium-resistant phenotype. The resistance is due to the high level of expression of human MT-II in those cells. The vector is maintained in the cells as a free replicating plasmid, present at about 10--15 copies per cell. Transcription of the episomal human MT-IIA gene is initiated from its authentic start sites and is regulated by the level of cadmium in the growth medium. The presence of the human MT-IIA gene allows the BPV replicon to function even though it is ligated to an intact copy of pBR322. Due to the presence of plasmid origins of replication and dominantly acting selective markers functional in both Escherichia coli and mammalian cells, pMTII-BPV can be used as a shuttle vector.