Replicon termination in E. coli

Termination of replication is regulated in E. coli.

The two replication forks that originate from the single bidirectional origin of replication on the E. coli chromosome meet about half way around the circular chromosome.
The region where the forks meet has been transferred to an extrachromosomal element for study. When the region is placed asymmetrically with respect to the origin, the fork with the shorter length of DNA to replicate stops in this region.
Deletion and mutation analyses reveal two pairs of termination signals, one for each fork. The signals are called terminators.
Inversion of a terminator abolishes its function for the fork it was terminating.
Terminator activity requires the function of the tus gene. A factor encoded by the tus gene binds the terminator sequence.
A target site for site-specific recombination is also present in the region between terminators.

Terminators function unidirectionally and arrest replication as the fork passes through those sequences.
The second sequence of the pair is thought to act as a back-up, in case the first fails.
When two forks collide, either between the two pairs of termination signals, or at a termination signal, the forks are annihilated. Concatenated circles should result.
Resolution of concatenated circles is a likely function of site-specific recombination.

Why the tus protein acts directionally has conflicting explanations derived from crystallographic structures (ref).
The tus factor inhibits DNA helicase activity, and is thought by some therefore to inhibit the progression of the replication fork.
Four additional terminator signals have been found beyond the sets half way around the chromosome from the origin. They are in the correct orientation to serve as back up terminators should termination at the principal sites fail.
Whether termination of replication is also regulated in nucleated cells is debatable.

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