The Rolling Circle for ϕX DNA Replication, II. Synthesis of Single-Stranded Circles

ϕX-infected cells have been allowed to incorporate tritiated thymidine late in the phage life cycle when single-stranded circles are the product of DNA synthesis. Virtually all of the radioactivity is recovered in a continuum of actively replicating viral DNA molecules. These molecules are termed rolling circle intermediates because they are characterized by three structural properties. They possess positive strands that are longer than the length of a mature viral genome, and negative strands that are covalently closed single-stranded circles. The 3′ termini of the long positive strands lie upon the template rings, while the 5′ ends are free in solution.