Eukaryotic expression vectors have been used successfully in viral LT-expressing cell lines (ie. COS) to clone cDNAs encoding proteins that can be detected through their bio-activity or reactivity with specific antibodies. Since Chinese hamster ovary cells (CHO) have been used extensively for the isolation and characterization of somatic cell mutants, we felt it would be an advantage to develop an expression cloning system in CHO cells.

We have modified the eukaryotic expression vector CDM8 by replacing the polyoma and SV40 origins of replication with the 427bp non-coding region of the Syrian hamster papovavirus. Wild-type CHO cells and the CHO glycosylation-mutant Lec4A were transfected with plasmids bearing the early genes of either polyoma virus or hamster papovavirus in order to establish stable, LT antigen-expressing cell lines designated CHOP or CHOH, respectively. CHOP cell lines expressing polyoma LT antigen supported efficient replication of CDM8, but replicated pMH poorly. Conversely, CHOH cells expressing the hamster papovavirus LT antigen supported replication of pMH, and at a lower efficiency, CDM8. Replication of CDM8 and pMH vectors were equally efficient in selected CHOP and CHOH cell lines, respectively and comparable to that of CDM8 replication in COS-1 cells. A bacterial β-galactosidase fusion gene inserted into the multiple cloning site of a CDM8 derivative was efficiently expressed when transiently transfected into CHOP and CHOH cells but not CHO cells since only the former supports autonomous plasmid replication. These results show that expression-cloning in CHO cells expressing either polyoma virus or hamster papovavirus LT antigens is possible using either the CDM8 or the pMH vectors, respectively.