Fig. 1.

Possible consequences of fork convergence at termination of E. coli replication. (A) Two replication forks with DnaB helicase encircling the lagging strand collide. (B) The two DnaB helicases may slipover the flush 3′ ss/ds ends of the leading stands of the opposite fork. (C) The two DnaB helicases may invade both leading strand ss/ds ends to form complementary 3′ ssDNAs that pair. SbcCD could then cleave the palindrome-like sequence, followed by RecBC/ExoI processing. (D) One DnaB invades a leading strand of the opposite fork. The displaced 3′ ssDNA is susceptible to a 3′-5′exonuclese, forming a primed junction for polymerase extension back to DnaB. The resulting hairpin/palindrome is processed by SbcCD and RecBCD as in C.