Replication terminator protein(Tus)/DNA complex
DNA replication proceeds in three steps: initiation from some origin, elongation of the DNA chains, and the termination of the replication. In all of these steps, a number of proteins work together. Initiator proteins bind to replication origins, and DNA helicases unwind the DNA helices. Single-strand DNA (ssDNA) binding proteins bind to the unwound DNA to prevent it from re-winding. The DNA polymerases extend DNA chains at the replication forks, at which the nucleotides are replicated. Replication-terminator proteins functions to arrest the polymerase and terminate the DNA replication in only one direction. The bidirectional DNA replication in the circular chromosome of Escherichia coli is initiated from a single origin. Two replication forks run in opposite directions and terminate at a point opposite to the origin. In bidirectional replication, two mechanisms have been proposed for the inhibition of the DNA replication fork by the terminator protein (Tus). One is that the Tus protein interacts with other specific components in DNA replication and inhibits the unwinding of double-strand DNA (dsDNA) by the DNA helicase, DnaB (model 1). The other is that it physically blocks the progression of the DNA replication forks. The three-dimensional structure of Tus bound to the terminus-site DNA (Ter) has shown which model explains the uni-directionality of the replication-termination (model 2).
The Tus consists of two domains and accommodates the Ter molecule in the central cleft formed by the domains. The beta-strands of the Tus recognize the phosphate groups of Ter by polar interactions from the opposite side of the replication progression only. Therefore these interactions biased toward one direction. The alpha-helices of the Tus, moreover, are concentrated on the fork-blocking side, and clasp dsDNA to protect it against the activity of DnaB. Thus, because the Ter sites are bound to the Tus by polar interactions in only one direction, a passage of the replication fork at the Ter site in only one direction is arrested but passage in the other direction is allowed. As described above, the complex structure between Tus and Ter has shown that the polar fork-blocking mechanism (i.e., model 2) is predominant in the replication-termination process in only one direction.
Protein Data Bank (PDB)
Kamada, K. Horiuchi, T. Ohsumi, K. Shimamoto, N. Morikawa, K.; "Structure of a replication-terminator protein complexed with DNA."; Nature; (1996) 383:598-603 PubMed:8857533.
author: Yuko Tsuchiya