Abstract
Simian virus 40 large tumour antigen (T antigen) is shown to catalyse the formation of duplex DNA from complementary strands in specific conditions. The activity is dependent on an excess of unspecific double-stranded DNA and seems not to function by T antigen mediated destabilization of secondary structure. Rather, protein-protein contacts between T antigen molecules appear to be involved. Protein-protein interactions between T antigen molecules bound to physically separated DNA sites are also demonstrated by the formation of specific DNA loops and by cyclization of DNA molecules with 3'-extended single-stranded ends where T antigen specifically binds to the single-stranded/double-stranded junctions. The relevance of these properties for T antigen functions in DNA replication, transcription and(or) recombination is discussed.