Early B cell factor (EBF) was identified and cloned as a transcription factor expressed specifically in B lymphocytes and adipocytes. This protein was also identified as olfactory factor 1

We generated a series of COOH-terminal truncated simian virus 40 large tumor (T) antigens by using oligonucleotide-directed site-specific mutagenesis. The mutant proteins [T(1-650) to T(1-516)] were expressed in insect cells infected with recombinant baculoviruses. T(1-623) and shorter proteins [T(1-621) to T(1-516)1 appeared to be structurally changed in a region between residues 269 and 522, as determined by increased sensitivities to trypsin digestion and by altered reactivities to several monoclonal antibodies. These same mutant proteins bound significantly less nonorigin plasmid DNA (15%) and calf thymus DNA (25%) than longer proteins [T(1-625) to T(1-708)]. However, all mutant T antigens exhibited a nearly wild-type level of viral origin-specific DNA binding and binding to a helicase substrate DNA. This indicated that binding to origin and helicase substrate DNAs is separable from about 85% of nonspecific binding to double-stranded DNA. As an independent confirmation that a region distinct from the origin-binding domain (amino acids 147 to 247) is * Corresponding author. origin-binding domain (residues 147 to about 247) (2, 49-52, 61) has been the best characterized. The maximal p53binding and helicase substrate-binding regions map to residues 272 to 517 (46) and 131 to 517 (75), respectively, and the ATP-binding region maps to amino acids (aa) 418 to 528 (4, 5). The first N-terminal 121 amino acids have been implicated in transactivation (57, 79), and a small region (residues 105 to 114) is responsible for binding to the Rb protein (13).One activity of T antigen that is less well characterized is its ability to bind nonspecifically to double-stranded and single-stranded DNAs (7,38,56). The exact function of this activity is not yet clear, but it has been implicated in origin-specific DNA binding (51). It may also be involved in subsequent steps during viral DNA replication, in the induction of cellular DNA synthesis, or in the activation of rRNA genes. We have mapped nonspecific DNA binding to a region between aa residues 131 and 517 (75). Recently, we showed that at least 5 residues (mapping between positions 149 and 203) within the DNA binding domain were important in binding nonspecifically to doubleand single-stranded DNAs (51,52). All mutations in the domain which significantly reduced nonspecific binding also prevented originspecific binding (although the converse was not true). This suggested that T antigen first binds to virus DNA nonspecifically and then locates the origin sequences.Several years ago, Prives et al. (44) reported that a 56-kDa adeno-SV40 hybrid protein (containing T-antigen sequences from about residues 252 to 708) bound calf thymus DNA but could not bind specifically to the SV40 origin. A smaller, 45-kDa protein, with T-antigen sequences from about residues 336 to 708 was unable to bind to any DNA. These 5443 Vol. 66,No. 9