subtilis Ter site (discussed recently in detail in reference 44), while as described below, Tus binds as a monomer to a full (asymmetric) E. RTP binds as a dimer of dimers to two symmetric half-sites within a full B. coli ( 71), the two terminator proteins are completely unrelated in sequence and in structure and bind their respective Ter sites in quite different ways ( 85, 172). subtilis were initially thought to be similar to those from E. subtilis termination system is the only other one where the molecular structure of the replication terminator protein (RTP) in complex with a cognate Ter site is known and the only one where structures of both the free ( 27, 134) and DNA-bound ( 172) forms of the protein have been determined. coli has developed in parallel with work on the mechanistically related system in Bacillus subtilis ( 26, 169). coli and closely related eubacteria, understanding of termination in E. Although discussion here is limited to the system as it has evolved in E. Several aspects of replication termination ( 7, 13, 19, 26, 58, 67, 78, 108, 120, 145, 153) and Tus- Ter interaction ( 85, 170) have been reviewed previously. coli, and then we examine in molecular detail the current hypotheses concerning the mechanism by which interaction of the replication terminator protein (Tus) at Ter sites leads to polar arrest of advancing replication forks. Here we give a historical overview of the development of this model for the process of replication termination in E. All Ter sites are oriented so that the replication forks can travel in the origin-to-terminus direction but not the opposite direction. Positions of Ter sites and the tus gene on the E. We describe some further experiments and insights that may assist in unraveling the details of this fascinating process. The evidence suggests, therefore, that the termination system is more subtle and complex than may have been assumed. We conclude that while it is possible to explain polar fork arrest by a mechanism involving only the Tus- Ter interaction, there are also strong indications of a role for specific Tus-DnaB interactions. This review integrates three decades of experimental information on the action of the Tus- Ter complex with information available from the Tus- TerA crystal structure. One proposed mechanism involves a specific interaction between Tus- Ter and the helicase that prevents further DNA unwinding, while another is that the Tus- Ter complex itself is sufficient to block the helicase in a polar manner, without the need for specific protein-protein interactions. The Tus- Ter complex acts by blocking the action of the replicative DnaB helicase, but details of the mechanism are uncertain. A replication fork can pass through a Tus- Ter complex when traveling in one direction but not the other, and the chromosomal Ter sites are oriented so replication forks can enter, but not exit, the terminus region. The arrest of DNA replication in Escherichia coli is triggered by the encounter of a replisome with a Tus protein- Ter DNA complex.
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