Tus-Ter complex

Termination Eventually, the two replication forks of the circular E. coli chromosome meet at a terminus region containing multiple copies of a 20 bp sequence called Ter (for terminus) (Fig. 25-17a). The Ter sequences are arranged on the chromosome to create a sort of trap that a replication fork can enter but cannot leave. The Ter sequences function as binding sites for a protein called Tus (terminus utilization substance). The Tus-Ter complex can arrest a replication fork from only one direction. Only one Tus-Ter complex functions per replication cycle—the complex first encountered by either... 

So, when either replication fork encounters a functional Tus-Ter complex, it halts the other fork halts when it meets the first (arrested) fork. The final few hundred base pairs of DNA between these large protein complexes are then replicated (by an as yet unknown mechanism), completing two topologically interlinked (catenated) circular chromosomes (Fig. 25-17b). DNA circles linked in this way are known as catenanes. Separation of the catenated circles in E. coli requires topoi-somerase IV (a type II topoisomerase). The separated chromosomes then segregate into daughter cells at cell... 

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