The simian virus 40 core origin contains two separate sequence modules that support T-antigen double-hexamer assembly

Using subfragments of the simian virus 40 (SV40) core origin, we demonstrat e that two alternative modules exist for the assembly of T-antigen (T-ag) d ouble hexamers. Pentanucleotides 1 and 3 and the early palindrome (EP) cons titute one assembly unit, while pentanucleotides 2 and 4 and the AT-rich re gion constitute a second, relatively weak, assembly unit. Related studies i ndicate that on the unit made up of pentanucleotide 1 and 3 and the EP asse mbly unit, the first hexamer forms on pentanucleotide 1 and that owing to a dditional protein-DNA and protein-protein interactions, the second hexamer is able to form on pentanucleotide 3. Oligomerization on the unit made up o f pentanucleotide 2 and 4 and the AT-rich region is initiated by assembly o f a hexamer on pentanucleotide 4; subsequent formation of the second hexame r takes place on pentanucleotide 2. Given that oligomerization on the SV40 origin is limited to double-hexamer formation, it is likely that only a sin gle module is used for the initial assembly of T-ag double hexamers. Finall y, we discuss the evidence that nucleotide hydrolysis is required for the r emodeling events that result in the utilization of the second assembly unit .