Replication Fork Breakage and Restart in Escherichia coli

Area of Science:

• Molecular Biology
• Genetics
• Microbiology

Background:

• Replication impairments lead to genome rearrangements via double-stranded DNA (dsDNA) ends at inactivated replication forks.
• Three key reactions in *Escherichia coli* (fork encounter with ssDNA interruptions, fork reversal, head-to-tail collisions) serve as models for dsDNA end formation.

Purpose of the Study:

• To review experimental evidence on the timing, location, and mechanisms of dsDNA end formation at replication forks in *E. coli*.
• To investigate spontaneous replication fork breakage in wild-type and *recB* mutant *E. coli*.
• To explore the enzymatic requirements for replication restart from intact versus recombination-intermediate inactivated forks.

Main Methods:

• Review of experimental evidence on replication fork dynamics.
• Analysis of spontaneous replication fork breakage rates in wild-type and mutant *E. coli* strains.
• Recapitulation of observations on replication restart mechanisms.

Main Results:

• Spontaneous replication fork breakage occurs in 18% of wild-type *E. coli* cells per generation, likely due to replicating nicks/gaps.
• In *recB* mutants, fork breakage leads to heritable chromosome terminus dsDNA breaks during cell division.
• Most inactivated replication forks (18% of cells) appear to remain intact, with the replicative helicase potentially persisting on DNA.

Conclusions:

• Replication fork breakage is a common event in *E. coli*, with distinct mechanisms for formation and repair.
• The majority of inactivated replication forks may remain intact, relying on specific restart proteins for reactivation.
• Different enzymatic pathways are required for restarting intact inactivated forks versus those involving recombination intermediates.