The two replication forks initiated from oriC (min 84) eventually meet at a termination region on the opposite side of the chromosome, triggering a series of events that lead to completion of chromosome synthesis and then chromosome separation. The movement of the replication fork within the termination region is arrested or at least forced to pause for a long time at specific termination sites (Ter sites). Ter sites are conserved 11 bp sequences that may be present in two orientations. When present in one orientation, they allow the replication machinery to pass through but when present in the opposite orientation they stop replication. E. coli has a total of ten Ter sites that are present in two clusters, each containing five Ter sites. The TerC, TerB, TerF, TerG, and TerJ block the progress of a replication fork that moves in a clockwise direction. Whereas TerA,TerD,TerE,TerI, and TerH block the progress in the opposite direction.
A protein called the terminus utilization substance (Tus) binds to the Ter sites. The Tus protein binds to Ter as a monomer with a very high affinity, ensuring the polar function of the Tus·Ter complex. When present in the proper orientation, the Tus ·Ter complex arrests the progress of the replication fork by interfering with DnaB helicase's ability to unwind DNA at the replication fork. The gene that encodes the Tus protein, tus, is located at minute 36.3 so Jies within the termination region. The physiological significance of the Tus · Ter system remains an open question because null mutants lacking the Tus protein grow normally under a variety of growth conditions. Bacillus subtitis uses an entirely different protein in place of Tus, one that acts as a dimer to bind to termination sites that are approximately 30-bp long. This difference is surprising because E. coli and B. subtilis replication systems are quite similar in other respects.
Two additional enzymes, topoisomerase Ⅳand recombinase, are needed to allow the newly formed sister chromosomes to separate. Topoisomerase Ⅳ allows the interlinked or catenated chromosomes to separate. However, it cannot help when an odd number of recombination events occur during the replication process causing the two chromosomes to become joined by covalent bonds and form a dimer. Dimerization presents a problem because the two sister chromosomes will not be able to separate and move to daughter cells. Recombinase acts at a specific site within the termination region called dif, converting the dimer into two separate daughter chromosomes.