Transforming functions associated with Epstein-Barr virus.

Epstein-Barr virus (EBV) transforms the human B-lymphocytes it infects into lymphoblasts that are competent to proliferate indefinitely in tissue culture [1]. The dividing transformed lymphoblasts carry multiple and complete copies of EBV DNA as plasmids [1]. No viral functions that are necessary for EBV-induced transformation have been identified and characterized to date. We have developed an assay that aids in the identification of viral functions needed to maintain the viral plasmid in transformed cells. The assay employs a plasmid vector that encodes resistance to ampicillin and can replicate in E. coli. It also encodes resistance to the neomycin derivative G418, so that its presence can be selected in mammalian cells [2]. Fragments of viral DNA that span the genome have been molecularly cloned into this vector. These recombinant molecules have been transfected into four cell types, and survivors to G418 have been scored. The DNA from one region of EBV gives a 10- to 100-fold higher rate of survival per microgram of added DNA than does the DNA from all other recombinants, as tested in cells that already contain EBV genomes. This recombinant fragment of EBV is maintained in an apparently unrearranged state as a plasmid and therefore carries at least those cis-acting viral elements necessary for plasmid replication.