Coat protein of cauliflower mosaic virus binds to ssDNA

Alkali-denatured cauliflower mosaic virus (CaMV) virions incorporated ssDNA, added exogenously, into multimolecular complexes during dialysis against a neutral buffer. CaMV coat protein binding to tracer DNA, assessed by gel electrophoresis and autoradiography, was highly cooperative as judged by the absence of intermediate-sized complexes. The incorporation of labelled BglII fragments of a plasmid containing CaMV DNA into complexes was prevented by the presence of 0.16 g/l unlabelled calf thymus DNA. Lower concentrations of competitor DNA allowed binding of some BglII fragments although preventing the binding of others. The self-annealing poly(dI-dC) was much less efficient than calf thymus DNA in preventing the incorporation of fragments into complexes, suggesting a binding preference for ss- over dsDNA. In addition, dsDNA, minimally cross-linked to prevent strand separation, was bound only weakly. End-labelled ssDNA fragments in complexes were partially protected against DNase I. The nucleic acid-binding activity of CaMV coat protein may be responsible for the organization of replication complexes, the precursors to virion particles.