Complex formation between the adenovirus DNA-binding protein and single-stranded poly(rA). Cooperativity and salt dependence

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Abstract
The complex formed between adenovirus DNA-binding protein (AdDBP) and poly(rA) was investigated with circular dichroism spectroscopy. The binding process was studied at a variety of salt concentrations, and the titration curves were analyzed according to the contiguous cooperative binding model given by McGhee and von Hippel [McGhee, J. D., and von Hippel, P. H. (1974) J. Mol. Biol. 86, 469-489]. The cooperativity factor .omega. of the binding process is low (.omega. .apprxeq. 20-30) and independent of the salt concentration. This in contrast to the binding constant K for which a moderately strong salt dependence is observed: .vdelta. log (K.omega.)/.vdelta.log [NaCl]= -3.1. The size of the binding site was consistently calculated to be about 13. We also studied the C-terminal 39-kDa fragment which is sufficient for DNA replication in vitro. Complex formation between this fragment of AdDBP and poly(rA) appeared to be characterized by spectroscopic and binding properties similar to those of the intact protein. Only, the binding constant in 50 mM NaCl is a factor of 5 lower.