Figure-8 configuration of dimers of S13 and phiX174 replicative form DNA.

Abstract Evidence from electron microscopy of the replicative form of S13 and φX174 DNA shows the presence of a “figure-8” configuration. This species consists of two monomer length and one dimer length circular strands in covalently closed circular form and containing a fused junction that divides the molecule into two equal circular segments. Its existence is supported by the demonstration that it is converted by digestion with the restriction endonuclease of Hemophilus influenzae strain Rd to α- and X-shaped forms that retain the fused junction, and by examination by electron microscopy in the presence of ethidium bromide, which eliminates tangling and accidental overlays of parts of the DNA molecules. Kgure-8s are present to the extent of about 5% of the dimers present in replicative form DNA. They are proposed to be intermediates in genetic recombination in S13 and φX174.