Mechanistic Aspects of Termination of E. coli DNA Replication

Abstract

Complexes of the Tus terminator protein with Ter DNA sites in the terminus region of the E. coli chromosome block replication forks when they approach from one direction, but not the other. The first arriving fork is thus trapped between oppositely oriented Ter sites, where it stalls, awaiting the arrival of the second fork [1]. Two mechanisms have been proposed to explain polarity of action of the Tus‐Ter replication fork block. One involves specific interactions of the replicative DnaB helicase at the fork‐blocking (non‐permissive) face of the Tus‐Ter complex [2]. The other [3] depends on strand separation by DnaB at the front of the replisome: approach of the replisome from the permissive direction leads to displacement of Tus from Ter, while approach from the non‐permissive direction results in formation of a remarkably stable “locked” complex. “Lock” formation requires a conserved cytosine residue in Ter, which moves 14 Å from its normal position to bind in a cytosine‐specific pocket on the surface of Tus. The roles of the two mechanisms in determining polarity of action of the Tus‐Ter block will be reviewed.