The cellular proteins that bind specifically to the Epstein-Barr virus origin of plasmid DNA replication belong to a gene family.

This laboratory has previously reported that a tumor-promoting phorbol ester, phorbol 12-myristate 13-acetate ("12-O-tetradecanoylphorbol 13-acetate"), induces BJAB cells, a Burkitt lymphoma cell line, to express cellular proteins that bind to the origin of plasmid DNA replication (oriP) of Epstein-Barr virus. These oriP-binding proteins interfere with the EBV-encoded nuclear antigen EBNA-1, which binds to oriP in Raji cells. To further characterize these proteins, a lambda phage expression cDNA library made from phorbol ester-induced BJAB cells was screened for fusion proteins which bind to oriP. Two recombinant phages containing sequences encoding beta-galactosidase fusion proteins and designated lambda-OBP-1 and lambda-OBP-2 were identified. lambda-OBP-1 and lambda-OBP-2 contained 0.43 kbp and 0.61 kbp of BJAB cell cDNA, respectively, of which 395 bp were shared. Using lambda-OBP-1 as probe, two cDNAs of 1.4 kbp and 1.2 kbp, designated OBP-1 and OBP-2, respectively, were isolated. These cDNAs also shared the 395-bp sequence at the 3' end. With these cDNAs as probes, Northern blot analyses of mRNA from BJAB cells gave 1.4, 2.4, and 3.4-kb bands, but a Southern blot of human genomic DNA revealed one band. It is likely that the oriP-binding proteins were derived from spliced mRNA(s) of a gene family.