The role of Ter sites in control of replication termination in E. coli. (a) Ter sites are located in two clusters, halfway around the chromosome from oriC. Each cluster contains multiple Ter sites oriented in the same direction, but the two clusters have opposite polarity, indicated by the arrows. Tus protein binds to a Ter site, and Tus-Ter blocks helicase approaching from one direction and not the other. Replication forks can displace Tus and pass through the first Ter cluster they encounter, but they are blocked at the second cluster, which has the opposite polarity. (b) Replication forks of equal speed meet in the terminus region of the chromosome (left). The Tus-Ter system ensures that even replication forks moving at unequal speed will meet in the terminus region (right). (c) The Tus-Ter system prevents a replication fork from extending much beyond the halfway point around the chromosome, and thus ensures that the fork always moves in the same direction as transcription.