Termination of Replication

KEY CONCEPT
- The two replication forks usually meet halfway around the circle, but there are ter sites that cause termination if the replication forks go too far.
Sequences that are involved with termination are called ter sites. A ter site contains a short, ~23-bp sequence. The termination sequences are unidirectional; that is, they function in only one orientation. The ter site is recognized by a unidirectional contrahelicase (called Tus in E. coli and RTP in B. subtilis) that recognizes the consensus sequence and prevents the replication
fork from proceeding. The E. coli enzyme acts by antagonizing the replication helicase in a directional manner by direct contact between the DnaB helicase and Tus. Deletion of the ter sites does not, however, prevent normal replication cycles from occurring, although it does affect segregation of the daughter chromosomes.

Termination in E. coli has the interesting features shown in FIGURE 1. The two replication forks meet and halt in a region approximately halfway around the chromosome from the origin. In E. coli, two clusters of five ter sites each, including terK, -I, -E, -D,and -A on one side and terC, -B, -F, -G, and -H on the other, are located ~100 kb on either side of this termination region. Each set of ter sites is specific for one direction of fork movement; that is, each set of ter sites allows a replication fork into the termination region but does not allow it out the other side. For example,replication fork 1 can pass t hrough terC and terB into the region but it cannot continue past terE, -D, and -A. This arrangement creates a “replication fork trap.” If, for some reason, one fork is delayed so that the forks fail to meet in the middle, the faster fork will betrapped at the distal ter sites to wait f or the slower fork.
The trapping of the two replication forks in ter leads to transient over-replication. This must be followed by trimming and resection. The two forks must then be joined in a process resembling doublestranded break repair.
The situation is different in eukaryotes because of their linear chromosomes with multiple replicons.

FIGURE 1. Replication termini in E. coli are located in a regionbetween  two sets of ter sites.