Figure 1. Bacterial chromosomal and plasmid DNAs. (a) Electron micrograph of the chromosomal DNA of a single Escherichia coli cell (2.6 Mb) prepared by ammonium acetate disruption of the bacterial nucleoid followed by surface spreading in a cytochrome c monolayer. Molecules were shadowed with platinum palladium and photographed at a magnification of about ×1400. Reproduced with permission from Kavenoff R and Bowen B (1976) Chromosome (Berl.) 59: 89–101. Copyright © Springer-Verlag. (b) Electron micrograph of plasmid DNA (30 kb) purified from Staphylococcus aureus by equilibrium density gradient centrifugation. Purified plasmid DNA was spread in a cytochrome c monolayer, shadowed with platinum palladium and photographed at a magnification of about 32 000; it is reproduced here at an original magnification of ×14 400. The plasmid is about 1/100 the size of the E. coli chromosome. Note that both relaxed and supercoiled molecules are present (unpublished data).

Figure 2. Rolling circle replication: possible termination mechanism. Circular supercoiled plasmid molecule (A) is bound by dimeric initiator protein (RepC), which causes melting in the region of the double-strand origin (DSO) nicking site, resulting in the formation of a double stern-loop (cruciform) structure (B). This structure is nicked at a specific nucleotide site by one RepC subunit, which remains attached to the 5¢ side of the nick by a phosphotyrosine bond. DNA polymerase holoenzyme plus helicase and possibly single-strand binding protein combine to catalyse polymerization, using the 3¢ end of the nicked leading strand as the primer (C). At the end of the replication cycle, the nascent leading strand is extended a short distance past the DSO nick site, which becomes unpaired from the template (D). This allows RepC subunit A to perform a strand transfer, switching the two leading strand segments and thus circularizing the new leading strand and transferring the short leading strand extension to the 3¢ end of the old leading strand (E). The attached RepC, subunit B, then catalyses a second strand transfer, circularizing the displaced leading strand and leaving the short leading strand extension attached to the protein (RepC*), which is then released (F). Replication of the displaced leading strand is then initiated by the host RNA polymerase, using a unique palindromic single-strand initiation site (SSO) (G).