Interaction of DNA with DNA-binding proteins: protein exchange and complex stability.

The competition of the DNA-binding proteins I and II of Escherichia coli and of the phage fd DNA-binding protein for single-stranded DNA was investigated. Their roles in cells might be judged from their binding affinities to DNA and their mutual exchange in the DNA . protein complexes. Strongest binding on single strands was found for the phage protein. DNA-binding protein II displaced half of the protein I in the complex with single-stranded DNA when no double-stranded DNA was present. Protein-complexed single strands were protected against degradation. The protection is less pronounced for protein II which can increase the stability of the fd DNA complex with DNA-binding protein I against nucleolytic cleavage.