Mechanisms of polar arrest of a replication fork.

A DNA replication terminator sequence blocks an approaching replication fork when the moving replisome approaches from just one direction. The mechanism underlying polar arrest has been debated for years, but recent work has helped to reveal how a replication fork is blocked in Escherichia coli. Early work suggested that asymmetric interaction between terminator protein and terminator DNA contributes to polar fork arrest. A later study demonstrated that if the terminator DNA is partially unwound, the resulting melted DNA could bind tightly to the terminator protein, suggesting a mechanism for polar arrest that involves a locked complex. However, recent evidence suggests that the terminator protein-DNA contacts are not sufficient for polar arrest in vivo. Furthermore, polar arrest of a replication fork still occurs in the absence of a locked complex between the terminator protein and DNA. In E. coli and Bacillus subtilis, the bound terminator protein makes protein-protein contacts with the replication fork helicase, and these contacts are critical in blocking progression of the advancing fork. Thus, we propose that interactions between the replication fork helicase and terminator protein are the primary mechanism for polar fork arrest in bacteria, and that this primary mechanism is modulated by asymmetric contacts between the terminator protein and its cognate DNA sequence. In yeast, terminator sequences are present in rDNA non-transcribed spacers and a region immediately preceding the mating type switch locus Mat1, and the mechanism of polar arrest at these regions is beginning to be elucidated.