Figure 4.

Pausing of the replication fork leads to the generation of reversed fork molecules. (Top) Schematic representation of the 3.6 kb PstI–PstI restriction fragment of pPI21 containing both origins. DnaB helicase is blocked by the RNAII–DNA hybrid at the silent ColE1 origin and a RI containing an internal bubble is accumulated. The ends of the newly synthesised nascent strands at the silent origin separate from their corresponding parental strands and anneal to each other, forming a reversal of the fork. (Bottom) This structure resembles a Holliday junction, able to branch migrate. A similar scheme would apply if fork reversal occurred at the active origin.