Replication Termination inEscherichia coli: Structure and Antihelicase Activity of the Tus-TerComplex

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Abstract
SUMMARY: The arrest of DNA replication inEscherichia coliis triggered by the encounter of a replisome with a Tus protein-TerDNA complex. A replication fork can pass through a Tus-Tercomplex when traveling in one direction but not the other, and the chromosomalTersites are oriented so replication forks can enter, but not exit, the terminus region. The Tus-Tercomplex acts by blocking the action of the replicative DnaB helicase, but details of the mechanism are uncertain. One proposed mechanism involves a specific interaction between Tus-Terand the helicase that prevents further DNA unwinding, while another is that the Tus-Tercomplex itself is sufficient to block the helicase in a polar manner, without the need for specific protein-protein interactions. This review integrates three decades of experimental information on the action of the Tus-Tercomplex with information available from the Tus-TerAcrystal structure. We conclude that while it is possible to explain polar fork arrest by a mechanism involving only the Tus-Terinteraction, there are also strong indications of a role for specific Tus-DnaB interactions. The evidence suggests, therefore, that the termination system is more subtle and complex than may have been assumed. We describe some further experiments and insights that may assist in unraveling the details of this fascinating process.

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