The large tumor antigen (T antigen) is a genome regulation protein, coded by simian virus 40, that binds with high affinity to specific binding sites on viral DNA. The specifically bound T antigen is released from these sites in 0.2-0.3 M NaC1. Immunoprecipitation techniques were used to show that T antigen also dissociates in 0.2-0.3 M NaCl from mature viral chromatin but not from replicating viral chromatin. In fact, a considerable fraction of T antigen remains associated with replicating chromatin at NaCl concentrations as high as 1.2 M NaCl when most chromatin proteins, including histones, dissociate. However, T antigen binding to both replicating DNA and mature DNA is sensitive to intercalating drugs such as caffeine and ethidium bromide.We consider the possibility that the unexpectedly tight binding of T antigen to replicating DNA is related to the function that T antigen performs during viral DNA replication. A small fraction of the T antigen is associated with replicating and mature viral chromatin [13 -151. Below we describe experiments to show that the binding properties of T antigen in replicating and mature chromatin are remarkably different. T antigen remains associated with replicuting chromatin at concentrations of 1 M NaCl and above while 0.2-0.4 M NaCl suffices to release T antigen from nonreplicating chromatin as well as from T-antigen -SV40-DNA complexes formed in vitro. T antigen dissociates from replicating viral chromatin, however, in the presence of intercalating drugs, such as caffeine and ethidium bromide, although about tcntimes-higher ethidium bromide concentrations are required to release T antigen from replicating chromatin than from mature viral DNA. It is not known whether the unexpectedly tight binding of T antigen to replicating chromatin is due to a particular structure of a T antigen subclass, and/or, whether it is caused by the function that T antigen performs during chromatin replication.
MATERIALS AND METHODS
Cell Culiure Coizditioiis and Virus InfectionAfrican Green Monkey cells (line TC 7 ) were cultured in Dulbecco's-modified Eagle medium, containing 10 calf serum. Subconfluent cells were infected with 10 -20 plaqueforining units of SV40, strain SVS, per cell.When a uniform long-term label of viral DNA was required. infected cells were labeled with [3H]thymidine (100 pCiim1) for 1 h at 38 h after infection, followed by a 1-h chase with 10 mM nonradioactive thymidine. Pulse labeling was performed at 40 h after infection with 100 pCi/ml [3H]thyniidine for 1-9 min as indicated below.
Preparatioti of Viral NucleoproteiiiThe infected labeled cells were washed with cold Hanks' solution and resuspended in a low-ionic-strength buffer, termed lysis buffer: 0.25 M sucrose, 20 mM Hepes, pH 7.8, 0.5 mM MgCl,, 5 inM KCI, 1 mM dithiothreitol and 0.5% NP40 [I ti]. The cells were then gently vortexed and centrifuged to collect the nuclei. Viral nucleoprotein was eluted from the nuclear pellet in lysis buffer without sucrose and with 0.2% Triton X-100 instead of NP40. After several stroke...