Characterization of proteins encoded by the Epstein-Barr virus transactivator gene BMLF1

Epstein-Barr virus (EBV) encodes a gene product in the BamHI-M leftward reading frame 1 (BMLF1) that functions as a promiscuous transactivator acting upon many other enhancer-promoter combinations. This protein has been studied by producing a polyclonal rabbit antiserum directed against a LacZ-BMLF1 fusion protein that was synthesized in Escherichia coli. Western blotting was employed to demonstrate that this antiserum specifically detected the BMLF1 proteins in E. coli, monkey, mouse, or B cells transfected with this gene, and in EBV-positive B cells chemically induced to produce this protein. In these induced B cells, two major proteins of 50 and 60 kDa and several minor antigens were detected by these antibodies. Transfection of an expression vector containing the BMLF1 coding sequence resulted in the synthesis of only the 50 kDa proteins. These major products were phosphorylated in vivo and were localized to the cell nucleus. Only the larger 60-kDa antigen was specifically induced to be synthesized by a different EBV encoded transactivator, the BZLF1 gene product. Chemical induction of lymphocytes latently infected with EBV resulted in the synthesis of both the 60- and 50-kDa forms of the BMLF1 transactivator. Two major forms of this EBV-encoded transactivator have been detected. The 60-kDa form is presumably derived from the BSLF2-BMLF1 open reading frame while the smaller antigens, 50 kDa size, appear to be made only by the BMLF1 open reading frame. These two forms of the transactivator are differently regulated and the functional significance of this remains to be explored.