The Interaction of the A and A Proteins of Bacteriophage φX174 with Single‐Stranded and Double‐Stranded φX DNA in vitro

The binding of the bacteriophage φX174‐coded A and A* proteins to single‐stranded (ssDNA) and double‐stranded (dsDNA) φX DNA was studied by electron microscopy. The interaction of the A* protein with ssDNA and dsDNA was also studied by sedimentation velocity centrifugation. It was shown that the binding of the A and A* proteins to ssDNA occurs in a non‐cooperative manner and requires no or very little sequence specificity under the conditions used here. Both protein‐ssDNA complexes have the same compact structure caused by intrastrand cross‐linking through the interaction of protein molecules with separate parts of the ssDNA molecule. The A protein does not bind to φX dsDNA in the absence of divalent cations. The A* protein does bind to dsDNA, although it has a strong preference for binding to ssDNA. The structure of the A* protein‐dsDNA complexes is different from that of the A* protein‐ssDNA complexes, as the former have a rosetre‐like structure caused by protein‐protein interactions. High ionic strengths favour the formation of large condenwi aggregates.