29 DNA-Protein Complex and the Structure of Replicating DNA Molecules

Bacteriophage ϕ29, isolated from soil, is one of a series of smaller phages that infect the Bacillus species (Reilly and Spizizen 1965; Bradley 1967; Reilly 1976). The genome of ϕ29 is a nonpermuted, linear, double-stranded (DS) DNA molecule with a molecular weight of about 12 × 106 daltons (Anderson et al. 1966; Anderson and Mosharrafa 1968). Several lines of evidence suggest that ϕ29 DNA contains two molecules of a 27,000–31,000-dalton protein, one at each end of the DNA molecule (Harding and Ito 1976; Harding et al. 1978; Salas et al. 1978). First, ϕ29 DNA can be isolated from the phage particles as circular molecules and the circular molecules can be converted into linear monomer DNA molecules by treatment with proteolytic enzymes (Ortín et al. 1971). Second, from the difference in the buoyant densities of ϕ29 DNA and the ϕ29 DNA-protein complex in CsCl gradients, the percentage of protein bound to...